Preparing for the Last War

Generals are always preparing for the last war. Of course, if it ran exactly the same way as next-to-last war, there would be no point to make such a distinction, so by induction we can guess the next one isn't going to be quite the same either. Which is why the connotation is that this way they are not quite catching the train. But, hey, doing things that way won us the last war, so we're going to do them that way again!

Of course, any expectations not backed by practice would be speculative. Thus as a rule even people who actively prepare are never quite ready for what happens the next.

Film

 * The Trade Federation in The Phantom Menace invaded Naboo by blockading it on a planetary scale while continuously landing infantry and assault tanks with the expectation that The Slow Walk of their Battledroids would eventually wear down the natives. While this doesn't work because they were using a Keystone Army, they tried this again during the Clone Wars; with the main difference using warship to protect their carriers in orbit while the ground forces marches towards the enemy with tougher droids and newer technology with mixed results.

Literature
""The one on the end's on rockers, sir; must be the officers.""
 * The Discworld novel Pyramids has a scene involving this trope: two nations that are obviously expies of ancient Greece and Troy go to war with each other, each using the weapon that won the last war. Thus, the war begins with a dozen wooden horses arrayed on each side of the line of battle.

Web Comics

 * Schlock Mercenary had a moment with an uplifted ape [//www.schlockmercenary.com/2017-07-01 explaining] the concept to an alien new to the current galactic civilization.
 * Antihero for Hire backstory had a conflict called "The Unexpected War", in which Canada fought USA and annexed a few states… using genetically engineered dinosaurs because "the US was so busy protecting themselves against Weapons of Mass Destruction that they never made anything to protect against dinosaur attacks".

Real Life

 * The reason for the notion's popularity was World War I, when Napoleonic style of warfare died because machineguns made an open massed advance suicidal, and artillery made field fortification necessary, so everyone had to adapt quickly.
 * …and then World War II came, and those who expected another World War I were introduced to the concepts of mobile warfare and air war.
 * …except those who paid attention to Red October and wars in which the Soviet Union was entangled, including the Spanish Civil War: most high-ranked Red Army commanders participated in both last big wars, obviously considered their last and victorious big war, "the way to do it right", and hopelessly mired WWI abhorrent — and taught others this way. Which in turn led to promoting a very aggressive style that worked in highly mobile warfare, but was ill-prepared for adequate defence whenever "good offence" was not the best response.