Reality shocks I've seen personally:
1. Workspace/office not the one you were shown/told. This is the most common one, in my few data points. I think people know it's a factor in appeal of the job, so it's mentioned, but the followthrough doesn't always happen.
2. Person you interviewed with is leaving. (One of the people I hit it off really well with in interviews was a tech co-founder of the established company. When I arrive as a new Architect, he apologizes and says he had been looking forward to working with me (which is not something I normally hear), but he just got an offer for a supercomputer startup that he couldn't resist. Fortunately, some other great people remained, and I ended up working directly with one of the other tech founders. I supposed I might've been hired due to the company knowing that some people were looking to transition out.)
3. Job not as described. (I've mostly avoided this common pitfall, but once got bit by it. I was recruited as the Research Scientist who'd execute on an idea that was sorta outside the scope of two professors who'd come up with it, but that they wanted to see it done, and it was up my alley. But, subsequent to discussion, when I'd been told I'd be the sole person leading&doing, one of the professors' grad students decided to do the idea for their dissertation, and I didn't find out until the first day on the job. It got worse from there, but the saving grace was that an all-around great 3rd person was also an "assistant" on the team, so, morale-wise, I was able to complete the entire year-long appointment I'd agreed to. I declined when the PI renewed the appointment for another year, and both us assistants asked not to have our names on the papers. It was unfortunate, because we did some good work, and the PI was willing to help me get my own ideas funded, but I couldn't commit to another year of the accidentally-bait&switch project.)
4. Famous culture broken. (My anecdote wasn't a first-day reality shock for me, but first-year. A successful with-revenue early Web tech company in a university town had a locally well-known policy slogan about how you advance, which seemed much more widely quoted than the "don't be evil" of another company, and was repeated to me when I interviewed. Long story short, while I was working there as a Senior Engineer, someone in company leadership was willing to spoil culture and morale, to let someone's nephew (or similar) break the slogan. I went and talked with the head of engineering, who normally had amazing poise and charisma, but he had no good explanation for that one. This was when other dotcoms were hiring the then-rare experienced developers like crazy... but companies sometimes do things that employees don't understand, creating new reality shocks.)
5. Representative from new parent company, walking around with representative from offshoring company, talking to employees about what they do, for multiple days. A bit like in "Office Space", but at an advanced Web tech company. (Also, I was concerned when they started asking me about an elaborate 3D puzzle that someone had assembled near my workspace before I was hired, they seemed skeptical when I said I didn't know anything about it, and then one of them asked where they might buy such a 3D puzzle, which I happened to know, and I couldn't resist being knowledgeable/helpful, even as I suspected it was just a trick.)
Reality shocks like these are another reason I'd rather talk with prospective employers, and get as much sense of the team and company as I can, rather than get distracted with one-way leetcode whiteboarding.