One thing in RE4 that bothered me...

[Byzz]

The water sample puzzle is my favorite! Maybe the hardest puzzle on RE (I played all the games less the two outbreaks) but it’s so entertaining to resolve it that I left a save game near there just to be able to play it anytime I want to.

[TheOrganization]

I liked that puzzle even if it was annoying as hell.
Was good to have to sit and work things out, it took a while

[Itashious_boy]

what water puzzle?

[Byzz]

what water puzzle?

Water Sample puzzle, it's on RE3 in the dead factory, near to the end of the game.