I almost always adjust a pattern, usually for the following two reasons.
1. The pattern is a different size than I want. I'm sticking with lap quilts for the time being.
2. I made some sort of mistake and now I have to change everything up to hide it!
I often don't even use a pattern, just a basic design such as nine-patch. I add sashing and borders to make it fit my batting.
I'm a newbie and only do very simple stuff for now.