Basically it's any of the keys that have stabilizers underneath, which includes space, enter, shift, backspace and caps lock. Best thing to do is pull out all the keys surrounding these keys, then gently lift them up with your bare fingers. This is ESPECIALLY important to do with the space bar.
If you accidentally pull out the stabilizers, that's fine, it's fairly straightforward to re-seat them (at least on PCB...not so sure about plate-mounted). Just be gentle when you do it. In any case, accidents can happen.
