I've always heard that after ten years, you can start expecting things to go wrong with a house. My house is now ten years old and sure enough, my A/C started leaking freon. The radiator (shown here) had rusted to the point where a few pin hole leaks developed. Patching was theoretically possible, but wouldn't stop more holes from forming. It was at this point that I discovered the regulations on freon have changed and, because of that, they no longer make replacement coils for my A/C. The net result: I've now got an entirely new A/C. Thankfully, I was ready for this particular expense.
I've been doing my own version of Dave Ramsey's Financial Baby Steps since college. Every month, my bank automatically transfers a little money into an "Emergency Fund" account. I don't touch this account unless I run into an unexpected expense that I can't otherwise cover. (Like, say, needing a new A/C.) I had enough to cover the cost of the new unit. If you don't already have an emergency fund, I highly recommend setting one up with an automatic transfer. Like contributing to your retirement, very soon after you setup the automatic transfer, you'll adjust and won't miss it at all. It doesn't have to be much either. $50/month will net you $1,200 after two years. Just a little longer and you'll be able to replace an A/C without having to go into (or further into) debt.