Hilarious

I was having trouble with my computer. So I called Harold, the computer guy, to come over.
Harold clicked a couple of buttons and solved the problem. He gave me a bill for a minimum service call.
As he was walking away, I called after him, "So, what was wrong?"
He replied, "It was an ID ten T error."
I didn't want to appear stupid, but nonetheless inquired, "An ID ten T Error? What's that? In case I need to fix it again?"
Harold grinned.... "Haven't you ever heard of an ID ten T error before?"
"No," I replied.
"Write it down," he said, "and I think you'll figure it out."
So I wrote it down:
I D 1 0 T
I used to like Harold...