rotten egg dependency - noun phrase -  A service that a mission-critical application relies upon that nobody knows about but brings everything to a screaming halt when something happens to it.  In a sane world, said dependency should have nothing at all to do with the thing that just crashed.  Called this because it's as pleasant a surprise as a rotten easter egg at breakfast.  Best explicated by the following haiku:

It's not DNS
There's no way it's DNS
It was DNS