By not understanding how version control works. I've worked at places that had a surprising number of developers who would just merge things in ways that drop code from other developers.
I now work for a small business but in the interest of not getting bitten in the ass I don’t wish to give the name of my previous employer. It was a large defense contractor, but our values didn’t align so I moved on when I found another opportunity to put food on the table. I know that’s not a satisfying answer but I’m here for entertainment value and the opportunity cost might not be worth it. My main point was that even though they have the money they didn’t see the value in good software process.
All the time! We would leave bugs unfixed even if the fix was trivially easy because management felt productive listing it as a cost savings. Software maintenance was seen as a necessary evil.