I've encountered Requirements bugs due to the requirement being flat out wrong. Those are usually easy to find. Contradictory ones can hide until you try to implement them.

But one of my favorite tricky ones was an apparently very clearly written Requirement. It passed all the Requirements reviews, design passed design review, code passed, etc. Then the original writer of the requirement went to use the product and said "but that's not what I meant" Sure, I implemented exactly what he very clearly said (wrote) but that turned out to be very different from what he meant to say.

If all bugs were in code my job (as a programmer) would be so easy.

It's all the squishy human-worded requirements that cause the problems.

Maintenance too. Faulty maintenance routines can break stuff, anybody who's got a car knows that very well.