Short list, there is always something and I pick up incidental/incremental knowledge/skills here all the time-

• Got my Moose legs. Put some into production code for the first time.
• After almost 12 years of Perl finally learned to use unpack without having to ask for help.
• Improved my understanding of sysadmin issues considerably; semi-willingly.
• Learned to treat debugging methodically; as opposed to creatively, if that makes sense.