At the moment, I favour "I miscalculated". It implies that you did indeed calculate, but it just missed. Much better than "I couldn't be bothered thinking."

"I didn't notice the consequences of that particular check"

"I played the opening like a 1600"

The one I use most often is, Woops, didn't see that one coming. Followed closely by: this is not working out as I intended it to. 

"I am trying to do too many things at once."

That's a good one as it gives the impression that you have some sort of life outside of the interweb.

"I slipped!"

"I got an urgent phone call"

"My girlfriend distracted me"

"The baby was crying"

"Someone played my moves while I was in the toilet"

Well, anything that refuses to confess the fact that you had no idea what your opponent was up to the entire game

"I have just undergone a Vulcan memory wipe" ?

I went to stir the Stew and the cat jumped on my key board,,,,

"Lucky for you I managed to blow a winning position"

Pets - always good for an excuse. I tried to blame my dog's farts once but I don't think I got away with it.

Yeah, I've always liked this one. It's not just the arrogance of assuming the winning position, but the idea that "managing" to blow it all is something of an achievement in itself.

"Too bad I ran out of time."

Also, anything that refuses to recognize the effort and skill involved on the other side of the board

Someone paid me to lose to you....  $25.00 dollars and a Cheese burger.  Otherwise, I would have won in 10 moves.......

It was a deep positional sacrifice that didn't quite pan out. Either that of Alien invaders were disrupting my brainwaves.

I'm old.

If winning is what matters I respect the ones who fail.

Not really an excuse but still something I like to say.