A priori, it seems like a pretty huge issue, because it changes a sentence's meaning to its opposite. Fortunately, it's usually easy to notice. But then again, I obviously wouldn't know about any instances I haven't noticed.
Even something like Halloween, which apparently triggered this discussion, is not something most people, including kids, seem to be particularly familiar with. When I mentioned to a Chinese friend that we were celebrating Halloween last year, she advised me to be careful when inviting ghosts into my home. She was unaware that it is mostly a fun children's holiday where they dress up and get candy.
I'd blamed chinese factories needing more places to sell their plastic halloween gear, but now it sounds like it just comes down to US media saturation?
Then again, we should all be stealing more holidays from each other; a more syncretic world is a less boring one.
[My german teacher in high school said the best thing about growing up in southern germany was that they got all the holidays (both protestant and christian) off from school]
Sure, in translation one always has issues of sarcasm or irony, but I felt the tool was probably hallucinating more than being a useful work instrument.
Lagniappe: https://www.youtube.com/watch?v=CFCABnWlN8E
EDIT: and yes, I also prefer the older behaviour of translation programs, whose output was noticeably disfluent where it was poor instead of just bullshitting to stay fluent.