My wife came in and got me all excited and I hit the wrong button.....

I was drunk & cannot remember anything so no point asking me ?

My old standby excuse: man i was about to really get ready to crush you , if i had not gotten CHECKMATED you would have been in real trouble !

                                  Our favorite OTB excuse ! 

I pretend to act upset that I dropped a piece... so they take it happy i am distressed and losing. then I reveal it was a sac, and proceed to mate :)

The situation has developed not necessarily to my advantage

"the situation has developed not necessariliy to Japan's advantage"  From Emperor Hirohito's surrender announcement

I couldn't tell the difference between the queen and the king.

When I get crushed early: "Unlike you I don't need to remember opening lines"

Bobby Fisher did not make excuses when he lost as a kid growing up in Brooklyn, He just started Crying !! 

When I run out of time: "I made better moves earlier"

Mouse slip.

I have actually had that. But perhaps not as many times as I have claimed...

Chess? Oooohhh...I thought we were playing draughts!!

Oh baby oh oh oh oh oh oh oh ... YA ! What? I dropped a piece?! Sorry, someone Distracted me ...

i was calculating 10 moves ahead

I woulda won if coulda had white.

My dog ate my brain.

"I saw that coming when I misclicked ten moves back. I just wanted to make sure that you could see it."

Yup, I've heard that one before!

"I underestimated you." Heard it about a dozen time from one guy. Thrashed him mercilessly.

Being fairly honest I just say ''ooops'' ( but sometimes delay incase never seen... :)