The cursor sometimes displays as a bar, but in the wrong place - i.e. you can be typing somewhere else. Or the bar becomes stuck, and the cursor where you're typing becomes a small vertical rectangle - and it all doesn't quite work correctly.
This ambiguity over where the cursor really is, coupled with the (very nice) F8 feature, is quite dangerous: you could easily think you're going to execute one statement, when actually you will execute a quite different statement.