D'oh. My bad.

Rather an American use of language?

It was supposed to be reminiscent (SiC?) of the episode of South Park when a
load of aliens came to South Park and kept on killing a load of cows, and
when the kids confronted the aliens (who were subtitled), found out that
they were friendly, and asked them why they killed a load of cows, the
leader replied with something like "oh, that's his fault, he's new" and then
the new alien said "Sorry. My bad."

In hindsight, quite a specific bit of a random TV programme for me to expect
anyone to remember! :-)

Peter