I have pretty serious paranoia when it comes to meeting the parents of an SO because my first boyfriend's dad was so horrible to me. Some highlights:
Made fun of the way I speak, telling me I don't speak properly (whenever I used slang words from the local area...). He would hold his hand up and interrupt me and tell me to say it again but "this time speak properly".
Once interrupted a family meal to ask me, "Yourfavouritevagina, why do you come across as such a blond, vacuous barbie doll?" I was so shocked, and very young at the time (re: no spine) so I just sat there shocked. When I told my then-boyfriend he told me it was my problem and I couldn't take a joke...
Told me I would fail the subject he was tutoring me in (he taught at another school). I got an A, and I found out he got a B when he took the exam.
Ask me if I intended to raise my future children in the church, and when I told him if I had any I would raise them to choose what they believe he went on a rant about how "strange" I was.
Anyway it was so so horrible and a major contributing factor in dumping my ex's ass. Since then, I've been super paranoid about "meeting the parents".
The first time I met my now-SO's parents it was just... awkward but not terrible. They seemed nice, if not a little plain. Then my FIL started with his antics... MIL started not too long after. My FIL constantly made fun of my degree (arts, but applied to something I can get a job in) and told me "surely anyone can do that!". It was his favourite topic for ages and it really grated on me. I think my SO took it as "ah he just doesn't know what to talk to you about yet" but I wasn't really convinced. I just thought he was a total dickhead. Then the "major" incident happened when he smacked my ass (either with his hand/knee/foot, he kept changing his story over what happened) and when I loudly objected he said "Well, it's hard to miss that!" I was absolutely humiliated and I have never felt so degraded in my entire life. My SO finally saw the light of day and that his dad was being super out of order. Like, completely. It was a huge drama and the only reason I considered the apology was because I wanted to keep seeing my SO, and he lived with them at the time.
So anyway, I realised they were off people. MIL seemed to just want to keep the peace and that annoyed me a bit. She seemed angry at the time it happened but she was the leader in getting me round to apologise and she started off the entire conversation. Very very odd people.
MIL Highlights:
Luckily my SO is great with setting boundaries. It really helps keep the peace. I try not to worry too much about their visits but these days it just stresses me out. I would easily forget past incidents but every time I see them something else happens. Last time I saw them FIL went on a big racist rant and it was so embarrassing, we were outside!