I know, I start wierd threads.
This is the shortest sutta I know of:
AN 1.48 PTS: A i 10 (I,v,8)
Lahu-parivatta Sutta: Quick to Reverse Itself
"I don't envision a single thing that is as quick to reverse itself as the mind — so much so that there is no feasible simile for how quick to reverse itself it is."
Though each of these is considered an individual sutta:
http://www.accesstoinsight.org/tipitaka ... .than.html
But since they are so thematic, it's hard to consider them separate entities.
I wonder if there's anything shorter? Don't ask me why. I'm just curious. The Buddha typically isn't portrayed as being keen on brevity in the Suttas...