OK, a weird bug - Roger-X was editing his first instructable, and when he changed steps it suddenly changed from being logged in as him, to being logged in as me. No strange messages or useful screen-shots - it was just the bit at the top that changed.
The page also locked, so that in the end we had to shut down Firefox and re-start it.
Later, when I was looking at my comments, the login suddenly jumed back to Roger-X's. Again, no odd messages, and this time it didn't lock - I could access all his account quite easily.
Additionally - we are experiencing problems logging in and out - if one of us logs out, the other can't log in until we shut down FF and re-start (yes, we've tried flushing the cookies etc). We type in our username and password, and it just jumps back to the login option.
It's bleedin' annoying, folks.