Recently I've been assigned a bug to fix, which from my point of view, was actually a change request. After some investigation it turned out that this bug was caused by a defect in business requirements, but it was still considered as a bug.
I often see change requests being pushed forward masqueraded as bugs. I am just trying to figure out if there are any differences.
A "bug in requirements" means there was something wrong with how the functionality was originally designed.
A "change request" means that everything was defined correct, but the customer wants a piece of functionality changed/added.
To me, you fix a bug when you made a mistake while implementing the requirements whereas the requirements' writer creates a change request when he made a mistake in requirements that have been implemented.
It happens all the time.
We as developers generally get annoyed if we get our things thrown back at us as having bugs to fix where it wasn't our fault.
What to do depends on the situation and on the process. Unless you get your performance reviews damaged by those, just don't think of those much and implement those bugs/changes.