It is possible to get SourceTree into an infinite loop of authorisation prompts which continue even after the app has been force quit. Resolving requires 'ps -auxww | egrep -i sourcetree' and a kill -9 of all pids returned.

To reproduce:

1) Set up an SSH tunnel to a remote machine (may not be required to reproduce this, but explains why I was doing the next steps on port 80.
2) Open Bookmarks window.
3) Click 'Add Repository'.
4) Enter 'localhost:80' in Source Path / URL. (This is not the complete source path, but the user should be allowed to complete other fields and come back to this one.)
5) Click in Destination Path. An auth dialog appears (see attachment).
6) Click Cancel.

Expected behaviour: the dialog goes away and does not come back.
Actual behaviour: the dialog is recreated.

7) Quit SourceTree from the Dock menu.

Expected behaviour: SourceTree quits and takes the dialog with it.
Actual behaviour: NSBeep, and SourceTree stays running.

8) Force quit SourceTree.
9) Cancel the dialog.

Expected behaviour: the dialog goes away and does not come back, as the process that created is has been quit.
Actual behaviour: the dialog is recreated, as SourceTree has launched a background process that Force Quit does not touch. It is necessary to go to the command line as detailed at the start of this description in order to stop the relentless dialog onslaught.

The infinite loop should not occur. The user may realise they do not have the auth information, for example. In any case, it would be better to prompt once the user hits OK on the dialog, rather than as soon as the Source Path / URL field is defocused.