We all make mistakes, and I'll admit that I've accidentally done this in the past too. It's easily done, until you find a monitoring system that works well for you.
However, there's another reason why you should keep on top of your submissions, because we writers aren't the only ones to make mistakes. Two weeks ago another friend sent a text congratulating me on my short story, Flower Thief, which had just been published. I was puzzled. If my memory served me correctly I hadn't had a decision on the story yet. But, my memory not being what it once was, I decided to log onto my database and check it out. When I did so, my database still had that submission sitting at "Awaiting Decision". Perhaps I'd failed to update my records. Anyway, I went out and bought a copy of the publication to confirm that, yes, there it was in print.
Two days later, I had an email from the editor apologising profusely for the delay in notifying me and confirming the good news that my story was in the current issue and (more importantly!) that a cheque was in the post!
Whilst this was a lovely surprise (it made my Monday morning, that's for sure) it highlights a problem that could happen if any writer inadvertently submits the story to two different publications at the same time. Suppose both markets had published and there'd been a delay in notifying the writer? That could have been interesting, with both publications thinking they'd bought the first rights to publication!
So, make sure you have a robust system for recording your submissions and that you use it. And just remember that whilst writers lead busy lives and forget to update things, it can happen to editors too. It also suggests that it's useful having good friends who'll tell you that they've seen your published work!