I wanted to mine me some testnet Ether to get started but I didn't succeed, and I noticed that:
Once I start mining I can't seem to sync anymore (the last block was always somewhere under 2min, now I can get hours late)
I can't get it to stop mining. I have to close Ethereum Wallet to get it up to date again. When I press "stop mining" or Ctrl+Shift+M it doesn't do anything.