I don't know whether this is a recent development but I'm noticing it more and more now. You search for something and the snippet shows in bold certain words but you go to the page and the words aren't there. You go to the Google Cache and the text isn't there either, although it tells you your worlds have been highlighted.

Do you have an example search/ site?

D'oh, in this particular case I was being stupid, it was a website with hideable sections, so my Firefox search didn't find the word. I'm sure I've experienced this before though, but maybe I've been being stupid every time :).

When I can't find the search terms in the document, I do "View Source" and search for them there. Usually find them in a place that makes sense.