Dealing with failure, GM advice, no motivationals ;) Options

[Sengir]

I'm sure other GMs know this problem as well, so maybe somebody else has found a good solution for it...

Assume the standard run, players have met Johnson, done the legwork, created a plan without any serious problems, and now the fun part is about to begin...and then the face's cover is blown at the reception desk, somebody triggers an alert while climbing over the outer perimeter fence, or something like that. A stupid mistake, but realistically it should blow the mission, thereby putting a swift end to what looked like a fun evening so far.
Obviously, that's not the point of playing RPGs. My solution so far has always been to "adjust" rolls if necessary, or skip the dice altogether in favor of storytelling. But that's not really satisfying, either, because a game has to allow failure. And writing a complete plan B for every mission (which remains unused most of the time) just is too time-consuming...besides the fact that an impromptu assault on a corp facility in full lockdown should be suicide.


So, anyone good a good approach how players can screw up completely, but don't ruin the evening?