I've noticed that, if I get an exercise wrong, it keeps coming back until I get it right. If it's the last exercise in the set, I get it two or more times in a row.

UPDATE == More recently, I've noticed that as I move up the tree, there are a LOT of reappearances within my practice sessions. Earlier on, exercises only seemed to come back if I got them wrong... now they come back for no apparent reason, sometimes several times in one session. Maybe the tech folks need to do some tweaking.