I do recall the Lost World level of Sonic Adventure being a slight pain, but that was nothing compared to ANY level in Sonic the Hedgehog 06.

At first it doesn't seem to be that bad. Then you get to the part where Sonic says "Let's speed up!" and the game goes downhill from there.
You see, there is a part of almost every level where Sonic goes to his max speed. Sounds fun, right? The problem is you can't attack enemies, the controls are wonky (not that they ever aren't in this game) and if you run into a tree, you lose all your rings (or die).
Add an over-serious dramafest of a plot, horrendous acting, deaths that in no way could have been your fault and messed up character designs (seriously, what's up with
Eggman?) and you have a horrible mess of a game.
It's the only Sonic game with a save function (I've played) that I haven't forced myself to complete . . .