I discovered this issue when playing around the Alarm-Clock app. There might be something wrong in the Gaia logic. You can replicate this bug by either the following ways:
1. Set an alarm at A time. Reset the same alarm at B time. Delete this alarm. It'll still fire at A time and then crashes the app.
2. Set an alarm at A time. Wait to fire and snooze it. Reset the same alarm at B time. Delete this alarm. It'll still fire at (A + 5min) time and then crashes the app.
Directly assigning this to you if you don't mind. ;)
Thank you to find out the wrong logic to set an existed alarm.
I'll take over the issue.
The issue was fixed in Gaia.
Gaia pull request #3497 and landed(https://github.com/mozilla-b2g/gaia/pull/3497).
We can change the status to RESOLVED.