I was about to reply to a thread by Shamil, with a YouTube video embedded in it. I didn't want to have the video embedded in the quoted post, so I removed the "media=youtube" and [/media] tags. This I did by selecting the line with the BBCode and then pressing "Delete" - once to delete the line itself and then a couple of times to delete the extra space between the text above and below the deleted line. However, the text below showed some weird behaviour, jumping up and down. To summarize:
Delete # 1: BBCode Line deleted.
Delete # 2: Text below BBCode Line comes one line up.
Delete # 3: Text below BBCode Line goes back down.
Delete # 4: Text below BBCode Line comes one line up again.
Delete # 5: Text below BBCode Line comes one line up more where wanted.
This was with Firefox 3.6. I've made a screencast, since I don't know if the above description would clearly convey to you anything.