What is this WP message please?

On my single install WP (Version 3.5.1) I am seeing a message pop up a few times that I have never seen before:
- Are you sure you want to do this?
Please try again. -
I have not clicked on yes as I am not sure if this is some kind of undoable error?
Or have they just put in a double confirmation step now; although I don't see it all the time.
The last time saw it pop up I was trying to activate the wpmudev whmcs plug in.
Regards
David