Hate. It.
Especially when the author switches from past to present in the same scene, and back again. Squicks me really badly and takes me right out of the story.
But someone who would do that has no business calling himself and author.
As I said above, I build tense shifts into my narrative specifically to amplify suspension of disbelief by employing parallel construction and reiterative phrasing.
ETA: I.e., I use present tense to describe a thing in our world that the reader is familiar with, and then, at a later point in the book, I describe a thing in the fantasy world
also using present tense (because it "exists") and often using a sentence that has either an echoic alliterative or metrical structure to the earlier sentence describing the familiar thing. The brain hears it. It's music. You can't
not hear it, even if you don't consciously pick it up.
Tense shifts done from third omniscient can also be spectacular when done for comedic effect, especially in portal stories where it can illuminate the faux pas and the fish-out-of-water perspective. Douglas Adams did it in the
Hitchhiker's Guide series. Heinlein did it in
Glory Road. Spider Robinson did it in the
Callahan's Place series. There's a long history of it in SFF, but somewhere along the line we got this notion in our head that there's a "best way" to write a story that sells.
I have 320+ GR ratings and 120+ Amazon reviews on a book with multiple tense formations, and exactly two reviews that comment on the changing tenses. The vast majority of readers either didn't see it, or saw it and decided it works.
The fun part of this is that some of the readers who have contacted me believing my books are real have pointed to these tense shifts as me "slipping up" and accidentally stating that the other world truly exists. So there is such a thing as overselling your suspension of disbelief. (I studied sociolinguistics and the philosophy of language and worked in Army PSYOP. Soldiers I've worked with have since told me I should've seen this coming, but I honestly didn't think I was that good.)
There's a time and a place to play with tenses the same way there's a time and place to play with perspective. But like perspective shifts (especially first-person perspective shifts, which can make me
insane, and which just so many authors apparently don't seem to understand even exist), if the story is just jumping around between past and present for no reason because the author clearly doesn't understand how words go, I pop smoke real fast.