IMHO that would a rare to almost non-existant thing. Any sort of Mega Corp has an unending stream of IT work. There's no real need or desire to pump up the final bill. You'll make way more money in the long run coming in on time and under budget.
Sure they have an unending stream of work, but you rather still do far less work for far more money or rather; be able to bill far more hours for the same end result is preferable for profits. The art is then to still have a ravingly enthusiastic client at the end of the project. It's an art. Or a scam. As a techie I find it the latter but I admire how they keep the client happy, time and time again.