Internet Backdraft/Theater

Wicked:

Go to a British Wicked forum and start talking about your favorite actress for the role of Elphaba. Once the dust settles between the Idina Menzel fans and the Kerry Ellis fans, you may be surprised to learn there are many more actresses who have played the role.
You can do the same trick on most Broadway Wicked forums with Idina Menzel vs Shoshana Bean, or by suggesting that "Defying Gravity" is Stephanie J. Block's song rather than Menzel's.
Liking a Glinda who wasn't Kristin Chenoweth is always good for a fight.
Another British Wicked fight - Adam Garcia vs. Oliver Tompsett. And then Americans throw in Seb, Kris, and NLB. Loosely translated, it's an all-out battle royale for who the crap is the hottest Fiyero.
Shippings shouldn't be talked about if you don't want to start a riot. Is the Fan-Preferred Couple Glinda and Elphaba the one true pairing and fantastic case of Star-Crossed Lovers, or is it just shippers thinking everything is romantic? Is Fiyero's and Elphaba's lovely or just getting in the way? They're both canon so that adds fuel to flame.
- Related to it, but the sexualities of anyone. Is everyone Bisexual until proven otherwise or not? Is Glinda gay, or not?
Preferring either the book or musical, depending who you ask, is just asking for trouble.

Les Miserables:

Do you prefer Frances Ruffelle as Eponine? Yes? Then prepare for the Lea Salonga fangirls to rip you to shreds.
Do you prefer Lea Salonga as Eponine? Yes? Then prepare for the Frances Ruffelle fangirls to tear you to pieces.
Do you prefer Samantha Barks as Eponine? Yes? Then prepare for both sets of fangirls to turn on you and leave you splattered in a bloody mess all over the floor.
Do you think Marius belongs with Cosette? Yes? You will die.
Do you think Marius belongs with Eponine? Yes? You will die.
Do you love Colm Wilkinson as Valjean? Yes? You will die.
Do you think Colm Wilkinson wasn't all that great as Valjean? Yes? You will die.
Do you think Michael Ball is the Best Marius Ever and no one else could ever compare? Yes? You... will actually be all right on that one, come to think of it.
Say that Ruthie Henshall is the best Fantine? Prepare to be killed by the Lea Salonga fans.
Say that Lea Salonga is the best Fantine? Prepare to be killed by the Ruthie Henshall fans.
Say that Patti Lu Pone is the best Fantine. Prepare to be ignored.

Rent

Who is the best Mimi: Daphne Rubin-Vega or Rosario Dawson? Cue flamewar in 3... 2... 1...
- Suggest that Vanessa Hudgens was the best Mimi and you will either be torn to shreds or completely ignored.
Say Adam Pascal is the best Roger and prepare to be killed by Will Chase fans.
Say Will Chase is the best Roger and prepare to be killed by Adam Pascal fans.
Idina Menzel or Eden Espinosa, who is the better Maureen? Either answer will get you eaten alive by the other actress' fans.
Insult Jonathan Larson, the play itself, and/or any of its characters (especially Angel) and you will die.
Don't express any preference to the play or The Movie, unless you have a death wish.

Awards:

Wicked v. Avenue Q for the 2003 Tony is a polarizing topic, and it's not the oldest fight still going on.
Going further back, we have the Great Lion King Vs. Ragtime Debate of 1998.
1988's Phantom of the Opera vs. Into the Woods combines the "Who really should have won the Tony" and the "Andrew Lloyd Webber vs Stephen Sondheim" debates For Massive Damage.
There are still heated discussions on whether or not Hair should have won back in 1969 over 1776.

Other

Saying that you like ANY other version of Dr. Frank N Furter other than Tim Curry. Flaming will ensue.
Guaranteed Backdraft bait: Bernadette Peters, Mandy Patinkin, Patti LuPone, Stephen Sondheim, Andrew Lloyd Webber. Express either liking or disliking them and the other side will start flaming. For an evenbigger flamefest, say they're OK -- not great, not horrible, and you get to watch both the Haters and the Fans go up in a fireball together.
State a preference for either Stephen Sondheim or Andrew Lloyd Webber. If it's the former, expect to be called an elitist who can't enjoy entertainment, and the latter, a soppy, sentimental idiot. Expect the ALW fans to talk about how Stephen Sondheim doesn't write "hummable" music, and the Sondheimites to drag out the whole stealing-from-opera thing. However, saying you like both may defuse the situation as both sides stare at you in blank incomprehension.
On the Andrew Lloyd Webber note: mention the Phantom of the Opera sequel Love Never Dies. The very name is enough to erupt flames. Not hot enough for you? Don't just mention it, say that it's a good show.
- Say you think Cats was good on a theatre forum. Wear asbestos underwear -- you will be flamed.
- If that doesn't work, rinse and repeat, replacing the title with Spring Awakening.
The Phantom of the Opera:
- Sarah Brightman vs. Rebecca Caine.
- Michael Crawford vs. Howard McGillin vs. Colm Wilkinson.
- The Phantom: Heroic romantic figure, or creepy stalker?
Antistratfordianism (the hypothesis that William Shakespeare's plays were not really written by William Shakespeare) is a can of worms you don't want to even mention, much less open. Be careful, the mere mention of Kit Marlowe or Sir Francis Bacon is playing with fire on some Shakespeare forums as it can be constued as having antistratfordian implications.
Or Shakespeare's sexuality, for that matter. It's been less of a hot-button topic lately, but don't bring that up. Don't even bring up the sexualities of some of his characters.
Ask five musical theatre fans about the origin of the term 11 O'Clock Number. You'll get fifteen explanations and a sore head.
Like Tanz der Vampire? Like the Hamburg DVD? Prepare to be bombed with endless commentary on how it features The Worst Cast Ever.
- Magda: blonde or redhead? Village bicycle or virginal victim?