How about the chance that the players may have seen the cover of the adventure, or read an ad about it? This has gotten much better, but it seemed to happen a lot in 1e AD&D. I can specifically remember having a game break completely down because the DM was using the maps / cover of a module (T1, maybe? I'm remembering an incident with giant crayfish lurking in a pond, and another involving a statue that was actually a golem, but I'm not sure which one this was...) as a DM's screen, and the illustrations completely gave away the surprise encounter we where about to walk into. As players none of us could decide just how suspicious we should act and everything still be kosher. We finally had to show the DM the picture in question, and he quickly added in-game hints to bring our IC and OOC knowledge back into balance...