Comment 4 for bug 635854

Rpm too checks for disk space at the beginning of transaction, and quick test shows it basically working (and no, I don't think anybody tests it regularly).
The problem is any number of things can use up more disk-space while the transaction is in process: package scriptlets can generate arbitrarily amount of content which goes outside the any disk-space checks we do, and then there's the rest of the system - logs get written to, users doing things...