For my money, one of the stupidest things in the Hobbit "trilogy" are the were-worms. Cosmically gigantic monsters that appear for 3 seconds, could obviously kill every single soldier on the battlefield, and all they're used for is digging holes for some orcs. Plus they ruin all the action in LotR. If the orcs have were-worms, why not use them on Helms Deep? Why not use them on Minas Tirith? They're the worst. At least the other dumb stuff like the barrel race or Legolas using falling rocks as a staircase doesn't retroactively ruin LotR.