Debugging can sometimes be maddening. You can check and recheck your code yet it will absolutely not work as you expect. I recently hit the hidden invalid character bug. One of the dangers from copying html and pasting it as code.
The W3C CSS Validation Service solved the problem.
Copying and pasting often gives unexpected headaches. Usually it’s due to assuming that only one or two words need replacing from the copied text when in fact there are more or an incremental variable needs to be changed.
While I’d like to think copying and pasting saves time, in regards to source code and programming I think that it often enough ends being a frustrating pain and should be avoided.