I just transfered an ale to secondary - I thought it was done, but nope. When I was racking, it was at 1.022 - and in secondary, it started up again. Oh well. I don't usually check for fermentation done-ness with the hydrometer, too much effort. Hasn't been an issue for me until now.
I think, for me, it was the temperature - fairly cold in the basement, even though I put a brew-belt on the carboy, it was probably 66 or 68 deg. F or so. Basement is probably around 60, but the carboy is near an outside foundation wall.