It's very easy to make silly mistake when you are concentrating hard on the difficult stuff. I loose count when tightneing the cylinder heads for example, or just loose track of how far I've got.
Carpenters have a saying, measure twice, cut once :-k
I doubt 20 seconds will have made much difference as long as the parts hadn't been cleaned up at all, in which case there wouldn't have been much oil anyway.
Fingers crossed this time, BTW, if it's a TDi (you havn't said) then fit a genuine head gasket.