It depends on the likelihood of those instructions being noticed and followed. If you design something that looks a bit like a seat (so much so that you need to stick a notice on it that it is not a seat) then the odds are that notwithstanding the fact that you have put a notice on it, some people will try and sit on it and thus break it.

It is a bit like sticking big plant pots outside a bar and slapping on a "do not use as an ashtray" stickers. You know what the outcome is going to be when you put the pots there in the first place. Trying then to say that "well people just don't follow instructions" is a rather lame excuse IMHO for not fully thinking out the consequences of a design choice.