This can happen if you use the harpoon and then immediately buy a T weapon without cycling out of your #7 slot. Anything you buy for that slot will be automatically pulled out at that point.
Fix for that will be live soon, and since this actually causes a somewhat related bug that was mentioned in a different thread, we'll do what we are doing for that bug: When the fix is live this thread will be made public and we'll see if it's more than just the harpoon causing this problem.