We have cleaned literally thousands of ovens over the last four and half years and I have only had to replace the hinges of an oven, because the door wouldn't close properly, once and really it was difficult to see that it was our fault, the door hinge spring do slacken as they get older.
It took me about 30 minutes and when I changed the hinges the difference was amazing; you really had to put some effort in to open the door. So in all it is not a common problem at all and if you are looking to use this as an add on to your oven cleaning business I think it's a non starter.
