Anybody experienced this?

Yes so turned it off, did you have hidden posts in the topic 2 members posts were merged by a chance? didn't get round to making a bug report. Not tried it since upgrading to 4.10.2

I've just tested it and reproduced easily - I'll put a bug in for this now.

And now marked as fixed in 4.0.11

Confirmed that this doesn't happen anymore in 4.0.11 - behaviour is now that no merge takes place (correct, as the hidden post is in there between the 2 posts)

This *might* lead some users (ie the ones who can't see the hidden posts) to think that the merge functionality isn't working, but easily explained to them if needed.