Developers will sometimes intentionally create them either to entertain others or for themselves. Bugs of this nature must be followed up with the individual that created them. In short, a bug that reaches the end client is a problem, however the process itself is to blame, not any particular individual. Center on your process. If a customer discovers an important bug then this will highlight the importance of good testing and QA within your organisation.
Every defect is an improvement or educational opportunity for someone.
Capitalise on it. They are the brakes on the wheels of progress. It also depends on the nature of bug. If the bug is very clear then tester is responsible. But keep in mind testers are humans like developers and they can skip something.