Worst thing? Absuse LINQ and not in a fun sexy way.

Entity framework > check.
LINQ query > check.
Standard IEnunerable magic > check

The developer had decided that it would be a good idea to thread the enumerable all over the place and collapse it anytime they wanted access to the data.

I know it’s a rookie mistake a lot of people make, but it was some pretty core data that ended up being used all over the place, so it was a nightmare to correct and it really impacted performance.

Needless to say they felt very silly when we explained how LINQ was deferred.