Not really, specifically, I guess you hit a blood vessel large enough to bleed out at the cannula hole. I've had it a couple of times. Seems like rotten luck to get it twice so close together. Was it a certain site area it happened?
Actually, I usually get some signs of blood up the cannula. Which is rather freaky and I am not sure what happens then - does it suck up then get pushed out again, leaving some trace (I haven't had blockage warnings at that time). Either way, I guess there is trauma at the site, and blood is seeping or forcing its way to the surface whatever way it can.