The first thing newbie riders and kids do when hearing either "on your left" or a bell is look behind them to see what is happening, and in the process of looking behind, they ALWAYS pull to the left. Drives me nuts.
So, I pass them very cautiously, but I am amazed at the number of folks who pass at high speeds - especially passing kids on bikes. Kids are highly unpredictable, and I witnessed one getting mowed down by a rider (passing much faster than I would) while the kid - yes - looked behind him and pulled to the left when he heard a bell. Fortunately no injuries, but one scared kid.
Okay, Chris L. - you can now put in your standard comment about the uselessness of trails.