Lost in Space/WMG

Dr. Smith's original, eviler saboteur persona was a Split Personality implanted by his controllers, which upon failure melted down/reverted to the Large Ham Dr. Smith.
Much like an Evil Twin, EDS (Evil Dr. Smith) was implanted into the normal Dr. Smith via Brainwashing. When the mission failed, knowing he couldn't kill them, the programming did the next best thing: it broke down. But rather than revert to his original mind, the EDS warped into the cowardly, Smug Snake, betraying Large Ham we all came to know and love.

Think about it; the new, incompetently evil Doctor kept the Robinsons from returning to Earth for three seasons thanks to his bumbling. He had enough Pet the Dog moments to keep the Robinsons from killing him - though Major West came close several times to violating Thou Shalt Not Kill. All in all, Dr. Smith succeeded.

The Robinsons and Smith all died because of his sabotage, and they have been wandering a version of the afterlife patterned after their expectations to look like deep space.
This is supported by their encounters with members of the Norse pantheon in one episode, and another episode where they make it back to Earth only to be told by the people they meet there that they are dead.

Doctor Smith is dead and in Purgatory.
In this case, the Robinsons are hallucinations or projections imposed, either from within by his tortured subconscious, or from without by a Greater Authority to teach him empathy and concern for others. Given the number of times moral lessons come up in this journey and the number of times Dr. Smith fails to learn those moral lessons, it's likely he'll be in Purgatory for a long, long time.

The actress who played Verda was sleeping with the producer.
It's why she appeared more than once.

"1997". Yeah, right. This takes place in The Sixties, in the same universe as Questionable Content.
In QC, space travel is common enough that there was a space station in The Eighties, and it was a family posting (Hannelore was born on one); Robot's descendants are much smaller due to miniaturization and cheap enough that young adults with low-end jobs can afford them, but the telecommunications and ground transport technology is basically the same as in our present.

The pilot, "No Place To Hide", is a dream that Will or one of the other characters is having prior to lift off.
It would have to be before the dreamer knew there would be a robot on board and also before they met Dr. Smith, thus explaining their absences in the dream. If it's Will's dream, maybe he has latent psychic powers and that would explain why so many things in the dream, ex: the cyclops, getting lost, finding Priplanus, all came to pass in the actual first episode, "The Reluctant Stowaway". Why would Will be able to tell the future you ask? Because...

Will actually has latent Psychic Powers.
In the episode, "The Space Circus" Will is able to materialize objects out of thin air just by thinking about them, but only in the presence of the alien woman, Sinestra. This weird, unexplained talent combined with his high intelligence points to the fact that he may have other great powers hidden deep in his brilliant mind.

Debbie the Bloop is smarter than she appears.
No evidence to support this really, but anyone who has seen Futurama can attest to the fact that cute fuzzy aliens who act like innocent housepets are usually hiding something. Plus, what happens to all of Penny's other pets like the dog and the turtle? Perhaps Debbie is very, very possessive of Penny...

Judy is adopted/illegitimate.
Everyone's thinking it. Penny physically takes after John and Will physically takes after Maureen, but Judy looks like neither one of them. Either she was adopted at a time when the Robinson parents thought that they couldn't conceive only to be pleasantly surprised later on in life, or Maureen was getting friendly with the blonde milk man while John was working long hours down at the college early in their marriage.

A.P. Tucker's abduction story is true.
And he was abducted by the same aliens that later hijacked Zumdish's ship in "Two Weeks in Space".

Smith was working for aliens.
We assume that Aeolis Umbra is a foreign terrorist organization, but just how foreign? When Smith sees the bubble creature's derelict space ship floating there in "The Derelict", he says out loud that he thinks it is his superiors come to rescue him. It could be that, in this future, spaceships are easy to come by for all countries, or perhaps, given the blase attitude towards the fact that aliens exist shown throughout the rest of the series, maybe Aeolis Umbra was an alien organization all along who used human spies within Earth's space organizations in order to stop Humans from spreading throughout the galaxy. Perhaps they are even somehow related to the Galactic Tribunal that put the Robinsons on trial in "Prisoners of Space" which was, incidentally, headed by a bubble creature.

Lost In Space takes place in the same universe as Mystery Science Theater 3000.
It is a fact that Joel Hodgson named his character Joel Robinson after the space family Robinson, so it is also conceivable that he is related to them as well. Perhaps he is John's cousin or brother. The two series are alike tonally, both being sci-fi comedies and having campy/corny aspects to them. This makes the MST3K episode where they watch "I Was A Teenage Werewolf" all the more funny, because it featured a young Guy Williams in a bit role.

Mr. Nobody is the "god" that Bender met on Futurama.
Considering he turned into a celestial entity at the end of the episode he was in, and also considering the similar voice, it is not hard to believe that the god like being that Bender encountered in the Futurama episode "Godfellows" was in fact Mr. Nobody. This means Futurama also takes place in the LIS universe. The timelines even add up.

Dr. Cronos is a Time Lord, not a merchant.
And he was exiled to that crazy Salvador Dali-esque landscape for sheer incompetence. He was given the fake title of Time Merchant and told that he controls the fates of millions just to keep him busy and out of the other Time Lords' hair.

Penny wasn't really hypnotised in "The Promised Planet".
She just saw an opportunity to shed her good, innocent girl persona for a little while and go-go dance on a pool table. You'd do the same thing if you were trapped with your parents 24/7.

Priplanus is a wasteland because of nuclear war.
The idea goes that a long time ago, maybe centuries, Priplanus was a thriving alien world with one centralized civilization ruled over by a princess. Eventually, certain members of society began to follow a new leader who became so powerful that he and his followers challenged the princesses rule. The two sects, the ones who followed The Leader and the ones who were still loyal to the crown, moved apart from each other and developed seperate societies. Eventually the hatred between the factions grew so heated with constant wars that one side (the princesses side most likely) developed nuclear technology. They quickly moved their civilization below ground and launched a missile to destroy the traitors. Meanwhile, The Leader began to move his people below ground too, but it was too late. The bomb hit and destroyed his followers and blasted his very soul out of his body, sending it into a mask where it would sit, waiting to regain a human body. The fallout over most of the planet was so great that the princesses people decided to stay underground and wait it out, thus they became a lost, forgotten civilization. All that was left on the surface as evidence of this once great world were some old ruins with skeletons inside. The radioactivity, meanwhile, began to cause mutations in the animals and other creatures of Priplanus, making them grow in some cases to giant proportions and even contaminating the fruit and other plant life. Perhaps the reason the Cyclamen feed on Deutronium is because it was a Deutronium bomb!

The film version exists in the Friends universe
Let's face it. Matt LeBlanc played his character as Joey would play him.