When the problem is "understanding what you are doing" it's not so bad. But often the problem is more like "understand what everyone around you has done". If th

When the problem is "understanding what you are doing" it's not so bad. But often the problem is more like "understand what everyone around you has done". If they shared an object across threads, then you can't do mutation without a lock. If they retained a reference into your vector, then you can't grow it. It's this sort of invisible UB at a distance that gets so hard to deal with as projects get larger, even between experts.