I was just thinking about this on my way to work this morning, because I realized why my blinkers get left on on occasion.
I was making a right hand turn, blinker on, and as I'm bringing the bike upright, there is a car in front of me in my lane who had just come out of a driveway. I am now needing to change into the left lane, but I'm already reaching for the cancel signal button, and I don't press it hard enough to actually turn it off. I wonder if I pressed it hard enough, so I glance down very quickly at the dash signal and it is off, so I proceed on my merry way. At the light 2 blocks later, I realize that my glance must have been during the off phase, because the blinker is still on.
I think my turn signal doesn't get cancelled when I'm too busy dealing with more important things.