Continuous Integration and Deployment
Having just discussed version control, it's time to announce which VCS mistake I see more often than any other - the mistake that's made by everyone (myself included), regardless of their experience or expertise. And the winner is…
Adding new files to a project but forgetting to add them to the repository.
I don't do this very often - maybe less than once per month. But whenever I do, when the other developers on the team synchronize their repositories, we're left in a situation where everything works for me, but they can't build.
If we're lucky, the error will report that the file wasn't found, and we can quickly resolve the problem. If not, there'll be some other error about a missing symbol or something that will take time to track down before discovering the root cause.
If only we had some form of robot that would see every check-in, grab the source code, and try to build the product. If it couldn't...