My current exclamation of choice. It's particularly apt when pointing out something small that has gone awry which could potentially contribute to a chain of events that will cascade into something huge going supernova (the banana peel on the stairway routine, see also: Rube Goldberg).

Also good for pointing out things so terribly, awfully, soul-crushingly bad that a sort of cute noise is needed to break the tension (the VP of Sales resignation followed by a short order of COO resignation with some CIO resignation on the side - an example taken from my recent history. Yes, I said gack!).