He started to wonder if teaming up with a close friend might not be a ghastly mistake.

He had known, of course, that he was making a ghastly mistake; but knowing it didn't help in the least.

He saw himself making some ghastly mistake that would cost his partner a lot of money.

This whole thing had been one big, ghastly mistake.

It had to be some kind of ghastly mistake.

This has to be some kind of ghastly mistake.

He was scared stiff, thought a ghastly mistake had been made.

It seems that he made some most ghastly mistake about his finances.

I have the feeling that this is all some ghastly mistake.

This could all be the result of some ghastly mistake or misunderstanding.