From what I understand, when a timer runs out, it vibrates once and then resets to 00:00.

• It should vibrate repeatedly until dismissed so you don't ignore it and have your water boil over or similar (you may be timing something very urgent)
• There should be some sort of message that your timer expired so if you look at it a minute late you remember what happened

I also think it should not clear the timer value, but maybe that would work best after implementing my idea (#390 (comment)) to save timer presets. It would then show something like 05:00 after your 5 minute timer ended, and you would press the back button to get to a screen that lists all your timers, similar to the Pebble timer app.