Feeling Tense; Not Feeling The Tenses

I've been reading fanfiction for over fifteen years now, and this is something I have never been able to understand. Why do so many stories flit between verb tenses from paragraph to paragraph or even sentence to sentence?

I can completely understand how easy it would be to make these kind of mistakes if you change a story from present to past tense or past to present, but all stories can't have been edited like that, right? So what gives?

I know my grammar has been far from perfect and I occasionally have a thing for repetitive sentences and histrionics (among other problems), but I can say with complete confidence that this is a mistake I've never made unless I was changing the tense of a story while editing it. I'm not trying to single anyone out and I'm definitely not thinking of anyone in particular. It's just that I've been wondering about this for years.

So, can anyone out there enlighten me? How is it possible to get your tenses wrong when you're just straight-up writing a story? Have any of you guys done it?