I was involved in a programming discussion today where I made some statements that basically assumed axiomatically that circular references (between modules, classes, whatever) are generally bad. Once I got through with my pitch, my coworker asked, "what's wrong with circular references?"

I've got strong feelings on this, but it's hard for me to verbalize concisely and concretely. Any explanation that I may come up with tends to rely on other items that I too consider axioms ("can't use in isolation, so can't test", "unknown/undefined behavior as state mutates in the participating objects", etc.), but I'd love to hear a concise reason for why circular references are bad that don't take the kinds of leaps of faith that my own brain does, having spent many hours over the years untangling them to understand, fix, and extend various bits of code.

Edit: I am not asking about homogenous circular references, like those in a doubly-linked list or pointer-to-parent. This question is really asking about "larger scope" circular references, like libA calling libB which calls back to libA. Substitute 'module' for 'lib' if you like. Thanks for all of the answers so far!