I don't get the "falling under the bridge" joke. Is it even supposed to be a joke?
Probably the Japanese saying "It's all water under the bridge" for resolving issues or forgiving past mistakes/conflicts.
He planned to deal with the issue of the extra cake by claiming it as a business cost and when he turned his head a car was diving under the bridge too.
It could also be a reference to the saying "sweeping it under the rug" since he was being a bit devious by doing the above, but that might be a stretch. Unless maybe part of the joke was a foreigner using idioms wrong?
