This ones well know so i won't bother explaining it too much.
The Battle of Cannae. That cocky bastard Varro thinks he can just march his legions in a crowded checkerboard formation? He doesn't think any of the following:
Makes perfect sense for the Romans to use their checkerboard, maniple based army. It's what enabled their acquisition of Greece five years after the Second Punic War. It was flexible and a pretty good way to use their forces.
-Paullus is my elder and therefore a bit wiser, maybe I should listen to him
Paullus wanted to delay the battle. The Romans weren't a fan of that, Fabius was massively unpopular for his strategies that actually worked, and Hannibal specifically baited Varro when it was his day to command the army.
-My men are so tightly packed together that they can barely move my arms
Hannibal's doing, technically. The Roman force was very concentrated on breaking his weak centre, which was gradually falling back and forcing the Romans closer together, all the while Hannibal's more experienced troops were pushing the flanks in and forcing the Romans ever closer, until their usually flexible formation became a blob of men being crushed together.
-Roman cavalry won't stand a chance against Numidian
They probably would. The Numidians were pretty much a diversion whilst the Hispanic and Gallic cavalry were involved in the heavier cavalry fighting.
-Hannibal has defeated all forces and consuls before me (all of which had larger armies and use almost the same strategy), maybe I should approach with caution.
None of which had larger armies. Rome had never fielded an army as large as the one at Cannae, an impetuous commander could easily assume that their 86,000 men could sweep anything from the field.
In a bigger sense, Hannibal is the failure for not using the victory to take Rome apart one stone at a time. It was the only way they'd win the war.
