When I tried to clone a gitolite repository which needs ssh key, SourceTree gets stuck with showing "Checking Source..." and does not provide any further information.
After I checked out that repository with git command and import it to SourceTree and tried to pull it, there was a "Full Output" checkbox and this time I finally could see what has happened: putty stucked at "The host is not trusted. If you trust this host, enter "y" to add the key to PuTTY's cache and carry on connecting." thing.

I think SourceTree should have "Full Output" checkbox at clone dialog or at least provide information about what's going on during cloning repository to let users take appropriate action to solve the ongoing problem.