We bought a house that didn't have cable or DSL in a somewhat rural area -- the sellers were using 3G, satellite Internet, and dialup (!). They assured us that the only reason they hadn't had cable installed was the high cost they'd been quoted for running cable on the property (understandable as the house is almost 1/4 mile back from the street), and the cable company concurred that they'd be able to do the work, "no problem," and quoted a fee that didn't seem all that bad in the end (we took it into account when negotiating the price of the house).
Nevertheless, we made the sellers agree to a contingency - cable co. would successfully complete the work (on our dime, of course) and hook us up before the closing, and the total amount wouldn't exceed what we had been quoted. If the contingency wasn't met, the sale would fall through and we'd get our deposit back.
That contingency clause relieved me of a lot of stress over the next few months. Everything turned out OK in the end, but it was touch-and-go for a while.
Cable co. accepted the order with no problem, did a survey I guess, called us and said they wouldn't be able to do it. They were persuaded (not sure how -- agent took care of it) to reconsider, but then came back with a quote that was at least five times higher than before. Cue lots of emails and phone calls. It was helpful having an agent who lived in the community. In the end a local contractor dug the trench for us at a very reasonable price, cable co. came down on their estimate, and the work actually was completed at just under the original quote. The dirt was literally being shoveled back over the trench on the day of the closing.
As I recall it was actually a few days later that the cable guy came to do the inside wiring and "flip the switch," so strictly speaking the contingency (cable installed and working) wasn't satisfied by the closing, but we assumed we'd be OK at that point, and there were in fact no further hitches. We moved in and have enjoyed a (mostly) reliable, fast connection ever since. But we would absolutely have walked away, even though it was our "dream house," had we not been able to get the cable pulled. The contingency clause gave us the assurance that we could do that with no penalty. And, had it turned out that the work could be done but only at 5x the price, we could potentially have used the clause to negotiate a concession from the seller to cover the costs.