In Toaster you can specify a task to run when you start a build by typing a semicolon and the task name after the build target. If you type a task name that doesn't exit, the build starts but the UI shows an empty progress bar forever, even after the build fails. If you then restart Toaster, you can see the build failed and this error message:

Task do_foo does not exist for target zlib

The toaster_ui.log file says:

NOTE: Logging error 2: {'lineno': 198, 'message': 'Task do_foo does not exist for target zlib', 'pathname': '/home/yocto/master/bitbake/lib/bb/msg.py', 'build': <Build: 2 test invalid task (Release master (master), BBV master (Branch: master)) zlib>, 'level': 3}
ERROR: Task do_foo does not exist for target zlib

This happens in both master and toaster-next.