Mine does the same thing. I just get slightly annoyed for a few minutes and then ignore it. It happens most often when the keys are lagging way behind my typing speed, typically when the phone has a weak service connection or when using the browser. Facebook has been known to be a big memory leaker.
Posted from my CrackBerry at wapforums.crackberry.com
I've seen it a time or 2 in .419 when i'm typing faster than my Storm can keep up with. the keyboard usually closes and i just open it back up and carry on.