I saw this once back in late 2001 on my way home from Buenos Aires to Heathrow. Young lady appeared in Club not long after take-off, was spotted and escorted back to economy (no WT+ then). She reappeared some time later, was spotted again, only this time she was more reluctant to move claiming to be unwell but she went back eventually.
I once made a fool of myself on Air France. The cabin was all the same but split into 3 sections by moveable dividers. I hadn't realised that, at the time, they had three different service levels (even on Paris/London) and moved myself from a row of 3 (all seats full) immediately behind the rearmost divider to the row in front which was completely empty. I got moved back.
Staff travel, as others have noted, can have some interesting effects. I can remember having a duty ticket back from New York to London in Club with my family in economy on staff ID80s. I went back to see how they were doing and, on my way back got challenged. I showed my boarding pass and was asked why I was in Club when they had a flight crew from a cancelled flight back in economy. I was a bit taken aback because, as I said to the crew member who challenged me, all I did was check in and accept the seat I was given. It may have been the same trip when on a London/Chicago flight which had been delayed and turned the connection we had into something rather short I asked if we could all sit in Club for the landing so we got off first - that worked but only from top of descent (which was all I had hoped for) and we made the connection.
Then there was my mum. I'd brainwashed her with the message that staff tickets were always liable to be denied boarding so when a crew member that she knew came to her economy seat and asked her to get her bits and follow her she was fully expecting to be offloaded. In fact she was then seated in Club; but only from Amsterdam to London!