prejoicing 

Celebrating something before it actually happens, and thereby jinxing it.

The crowd roared as the skillful shooter released his three-point shot; but when the jumper clanged off the rim, they knew they had doomed it by prejoicing.

prejoicing by Alpine McGregor May 25, 2012

And There was Much Rejoicing 

Phrase from the comedy classic, Monty Python and the Holy Grail. Because of its relatively obscure nature, it is most often used among geeks and hackers. When someone uses this phrase, it is customary for anyone in earshot to point their index finger upward and wave it like a flag in a circular motion, coupled with a very insincere "yaay" - another reference to the original scene.

A: I smote the dragon, and there was much rejoicing.
B: *twirls finger in the air* yaay.

And There was Much Rejoicing by progamer124 September 7, 2003