I had such a project that was making about $15k/mo at its peak -- it was me and my wife, barring a couple of spot contributions from contractors. It depended on data from a sole data source, a Fortune 100 company, which eventually cut us off by sending a C&D threatening to sue under the CFAA, Copyright Act, and various other statutes. They claimed we breached the contract entered automatically by using their site, as their Terms of Service states that "no automated or manual method" (i.e., no method at all) can be used to access their content and their footer states that use of the site constitutes agreement (this is called "browsewrap"). The way we accessed data was much lower-impact than the way our clients accessed the data previously and I'm fully convinced that we saved them tens of thousands in maintenance and bandwidth costs.
No other data source is capable of providing the content we needed. We were forced to shut down.
We knew this was a possible eventuality and our ToS explicitly disclaimed responsibility for it. Our site required users to check both a checkbox and click OK on a dialog box that served only to inform that they used the product at their own risk, nothing was guaranteed, and no refunds of any kind would be furnished. When we shut down, many angry users demanded refunds and issued chargebacks, often after months of successful use, despite the clear and unambiguous language which confronted them several times and required their affirmative assent before they were allowed to purchase anything.
Before we were forced to shut down, other people had caught on to the market and started copying us. We had about a year where we were in it by ourselves. Once serious competitors showed up, they ate our lunch by using a sophisticated spam network to promote their offerings, which were sloppily made by offshore contractors and far worse than our offering in every way, technical and aesthetic.
I refused to engage in similar tactics and felt righteous about it, but it sure cost us a lot of money. They somehow brokered deals with the niche forums that had blacklisted us from day one (or just outspammed their moderation capacity), afraid that we may eventually expand into something that would threaten them directly (which I now plan to do, some day). Perhaps we could've prevented the copycats by acquiring software patents.
These competitors pushed the envelope to the point where it became a visible PR issue and the F100 was forced to respond by C&D'ing every site that operated in the sector I launched and instructing their users to never use anything not distributed directly by the company itself again.
Now, about 16 months post-shutdown, I still get emails from business owners who depended on us begging me to turn the service back on, and saying that their business has been seriously hurt by our absence.
Some of the people in this thread evidently picked much better niches than I did.