For a reasonably smart guy, I can be a real moron sometimes.
Last night I was siphoning with a carboy cap (w/ racking cane shoved through).
I have an autosiphon, but I really like this method because it holds the racking cane steady, so I am free to do other things (like sanitize bottles) wihout worrying about the autosiphon sliding to the bottom of the carboy and sucking up a bunch of trub.
Anyway, I kept losing my siphon after getting it started by forcing air (through a sanitary filter) into the carboy.
Duh: I was putting the little cap back on the smaller opening of the carboy cap (where the air had been forced), which was causing a vacuum in the carboy as the liquid siphoned out and no air could get in to replace it.
My advice: don't do that.