What are reasons for a blow head gasket?

I'm thinking of buying a 2001 Ford Taurus with 89K on it. The craigslist ad says it needs a head gasket

It might have been a faulty part when built or it might have been overheated which warped the head. Replacing a head gasket is labor-intensive, plus it's a waste of time to go through all that work and expense and not have the surface of the head check for warpage. If it's straight, no problem. If it's warped, the head will have to be machined.

Run away if it is more than $150.