Mortgage subsidies
Freddie and Fannie Mae: monuments to the ideal that market forces can be dispensed with if you have access to the Treasury.
A large part of the current problem with mortgages is the cherished idea--decades old and totally bipartisan, by the way--that just because someone can't afford to buy a house doesn't mean he shouldn't buy one.
If someone is turned down for a mortgage it's always been seen as some horrible aberration that needs to be immediately corrected by government help. If we had done less of this, a lot more of us would be renting instead of owning (there are VERY few homeowners that haven't benefited from some government program or other), but maybe we wouldn't be sitting on top of a thinning bubble, either.