See the diagram below for the names I'm using. Ok so we are starting with an active comparator (called Comp) in the diagram linked to below.

http://i.imgur.com/Cob5y.png

Now if the Rear Source is activated the signal is passed through, UNLESS the Side Source is currently active Should that side source be active, it prevents the main signal from passing. Except with ONE BUG. Even with the Side Source active, a SINGLE PULSE of redstone signal will make it through to the output. This is sufficient to trigger TNT. So if you were depending on proper operation of the comparator to prevent TNT from exploding when the Side Signal (used for "disarming" the bomb) was present, it would STILL EXPLODE from turning on the Rear Signal, even when it was supposedly "disarmed".