One way to find a single bug causing a mysterious error message is know as "Wolf trap debugging". The Wikipedia article about Debugging describes it as follows:

Edward Gauss described this simple but very useful and now famous algorithm in a 1982 article for communications of the ACM as follows: "There's one wolf in Alaska; how do you find it? First build a fence down the middle of the state, wait for the wolf to howl, determine which side of the fence it is on. Repeat process on that side only, until you get to the point where you can see the wolf."

So, in the case of a Mudcat post, you'd start by making a safe copy of the proposed post somewhere else, such as a plain text editor. You'd then eliminate about half the code, being careful not to lose the enclosing HTML tags. If the error message recurs, the offending code is in that half; otherwise, it's in the half you removed. If you're careful enough in removing and replacing halves, you can find the offending code fairly quickly. You can then fix the code in the safe copy, copy it to the composition area, and try previewing it again.