But a "major" bug free software is always possible irregardless of its complexity.
No it's not. There are so many hardware/software configurations out there these days that it's impossible to test for all bugs on them. The only way to catch these potential conflicts is to release the game and deal with the various problems as they come up, through the release of patches. Computer systems are simply too varied, there's no such thing as a standard to test from, like there is with consoles.