For the last few days I have had a similar problem. I click the side button in the morning so my ringer is set to Silent Mode while I am at my desk (the vibrating is enough to jolt the dead).

The phone is not in the cradle, just to the power charger laying on the desk.

Even though it clearly says it in in Silent Mode, even though no alarms sounds, they just vibrate - the PHONE STILL RINGS.

Yes, I have ring by category - but I have used it this way since I got the phone in mid-January and only this week does this pop up.

Strange, but true.

ALSO...
Does anyone know a shortcut to simply turn the phone to vibe mode, but keep the alarms audible? Without going to Prefs and changing to Vibe Only there on the ringers? I forget then, since I don't have the silent bell on the top screen to remind me the ringer is off.