Which are common hard problems / patterns that one might find with Pig, and their solutions?

Which are common hard problems / patterns that one might find with Pig,
and their solutions?

Which was the hardest "small challenge" you solved with Pig?
The idea here was just to sum up many clever solutions in a single place,
as a reference for everyone.
Let's define "small problem" in terms of it's description. Basic blocks,
or patterns, that anyone could need / use eventually.