Another possible symptom is a merge conflict. Programmer A makes a change that adds a list of things out of one thing and checks in. Programmer B checked out

Another possible symptom is a merge conflict. Programmer A makes a change that adds a list of things out of one thing and checks in. Programmer B checked out the "one thing" version and made an update to it. They check in, and they now need to deal with a merge conflict into a list of things that they now need to expend mental energy understanding. Mistakes get made.