If you had an App Engine update go awry, you may need to roll it back. You will get a message like this when things are messed up and you try to “Deploy Application”:

*** Running appfg.py with the following flags:
--no_cookies --email=acct@gmail.com --passin update
Password for acct@gmail.com: Scanning files on local disk.
Initiating update.
2008-08-12 08:51:32,117 ERROR appcfg.py:1334 An unexpected error occurred. Aborting.
Error 409: --- begin server output ---
Another transaction by user acct is already in progress for this app and major version. That user can undo the transaction with appcfg.py's "rollback" command.
--- end server output ---
If deploy fails you might need to 'rollback' manually.
The "Make Symlinks..." menu option can help with command-line work.
*** appcfg.py has finished with exit code 0 ***

The solution (paths from a mac) is to running the command:

python /usr/local/bin/appcfg.py -verbose --no_cookies --email=acct@gmail.com --passin rollback simplelti

“simplelti” is the application directory – you need to run this where your application is a subdirectory.