This has got to be the best (and the worst!) thing that players do. All of a sudden they think of something plausible that just isn't covered in the session notes and your on a roll, desperately trying to cobble together an explanation or a course of action. For example, TGS, a constant storm going on. One of my players decides to stand outside after dark and plot where the lightning strikes the most. A bit of triangulation and the result is the location of all of the lightning stones!
Anyone else got any examples
all in good fun ofcause, and if need be I have no problem with the GM just cheating 'behind his screen' to allow the bad guy to use hindsight and ofcause have thought of that possibility before hand .... even if the GM hadnt ... ie. sometimes its fun to solve a complex mystery by being clever ...and sometimes it just ruins the fun if the players are too smart or too lucky at the 'wrong time'
You can always get them back on track.