I just did the same I made a mistake on my last jobs inventory and I literally Just started the job, and it was pretty big according to them and they were thinking of firing me for it but before they could I told them I resign to not have it on my resume that I got let go.  It was a honest mistake and I let the manager know and according to them it was a big mistake and they couldn’t just let it slide and since I was within my 90 days I could be fired for pretty much anything.  I’m glad I resigned before firing me nonetheless.