When answering questions here and when drawing my own complicated TikZ/PGF drawings, I've developed a fair number of "tips and tricks" for debugging the code. I'm sure that others have as well. I think it would be useful to accumulate a list of such, whence this question:
What are your tips and tricks for debugging TikZ/PGF code?
Note: in one sense this seems an obvious candidate for making Community Wiki as there is no single answer. However, I think that these tricks take some ingenuity and so deserve to gain their authors a modicum of reputation, whence I'm reluctant to make it a CW question (of course, I'll donate all my reputation gained from this question to a worthy cause). Once there are a fair few answers, I can see that organising them into a single CW answer would be a good thing, but that doesn't mean that the question should be CW. On the other hand, whilst in favour of it not being CW, I wouldn't be in anyway upset if the feeling was that it should be CW.