I'm in web development myself, my answers:
If money seems to be the only important thing, or you're being compared to India
I've frequently had discussions with potential customers that in the end boiled down to that they didn't like our rates, either because "competitor X is 10% cheaper" or "I could outsource this to India for half your price". On both cases I move into arrogant mode, and ask whether they like to invest in top quality for a high rate, or low quality at half the rate, taking twice as long to not deliver what you want. Never, ever, get into a bidding war with these customers if you don't absolutely need them, believe in your product and let them decide whether to choose end product quality over a bit of discount or not.
Same goes for if the client is requesting offers from more than 2 competitors - that's an absolute red flag for me. I've heard customers tell me they requested 9 offers, but no one can evaluate that many offers on quality. Customers like that only care about the price, so they'll be whining about every invoice you'll ever send.
If you don't believe in the client's business
If you don't think they have a chance to succeed, you should be honest about that. If they can't convince you otherwise then, steer clear. Don't ruin your own reputation by becoming the guy that 'ruined a company by sending them invoice after invoice for a failed product'. They will try to blame you for their failure, it's human nature.
If the client has no technical knowledge whatsoever
Again, this means fighting about every invoice if they don't understand the complexities of the product. Customers like this only see that Facebook works, not the thousands of man hours involved, and will expect you to replicate that in a few days 'because you know this kind of stuff'. They hire you to be the expert, but they should know at least enough to appreciate your work, or it's going to be drama at one point or another. They don't need to know everything about your job, just enough to appreciate why they're paying someone else good money to do it for them.