I cringe every time I hear a PC say, "I'll wait here." or basically anything that will separate them from the rest of the group of PCs. Even though, it might seem like a smart thing to do in game at the time in their minds, it means that they will sit there for an hour twiddling their thumbs while the other people play the game.
Sure, as a GM I can try railroading them along with there friends, but it seems really apparent that I'm trying to do that (my fault as a GM). I'm thinking about telling them out of game before every session to try not to separate unless I hint to them that it might be beneficial to do so.