If you’ve used the MyBB Merge System v1.6.1 or earlier please download the file attached here, upload it to your forum root, and run it by going to http://yourdomain.com/yourforumpath/icr.php for security purposes.
The reason for this is a potential security breach related to the “old db” password being stored in the datacache in plaintext form. Thanks goes to euantor & Malcolm for reporting this.
MyBB Blog 
I found this issue some months ago, euantor reported it here:
http://community.mybb.com/thread-90789.html
Isn’t it the same issue?
Ok, both are credited now. Thanks.
Only just realized the blog announcement, thanks for mentioning me. ❤
Dylan, it wasn’t euantor who found it, it was me. But I do not want credit, I just wanted to let you know that it had been reported a while ago 😛
I didn’t thank him for finding it, but for reporting 😛
But thank you too 😉
I had honestly forgotten all about that thread, so I’m glad you mentioned it. This is why I put things on the bug tracker as soon as I read a post now.