Now you know why.  Here’s the science, and they advocate tin foil hats too (well not quite but close enough).

Scientists decode brain farts
Up to 30 seconds before your goof, the brain starts acting abnormally

updated 5:14 p.m. MT, Mon., April. 21, 2008
We’ve all goofed up and flubbed up things we’ve previously done time and again.

It turns out the root of these brain farts may be a special kind of abnormal brain activity that begins up to 30 seconds before a mistake even happens.

The solution to such screw-ups could be a kind of mind-reading hat, a device to predict and even prevent mindless errors that can threaten lives.

link here

Hmm. I guess my brain acts abnormally all the time, then.
*sigh*

I’d assumed it was a sign of senility which has been the excuse I’ve been using since third grade. 

In truth, monotonous tasks over an extended period of time bore or ‘rock’ the brain which normally then pursues other, more pleasurable thoughts that drift into alpha waves.  Keeping beta waves on all circuits going in order to alertly perform repetitive tasks is more akin to deliberate torture (and that was my further explanation in third grade).