Mining error while mining between two local clients

[dnotestein]

I had two local clients with each having a local identity which had already been successfully registered (two clients could communicate via email). I added a second identity to one of the clients and tried to mine it. Some time after that, I saw in console log:
delegate through exception... it shouldn't do that!
t.prev == my->_cur_block.prev

also, it looks like it mined a block with age 1, just prior to this. I have the log for it still.