Thongs
 

My wife now has a variety of thong bikinis, but even the ones at the more conservative end result in regular partial (and occasional full) asshole exposure when she's lying on her front.

The ones with less coverage on the front also allow the odd pussy peak when she moves around.

What I'm unsure of is whether it's something that happens to everyone (but I don't notice as much on others) or whether it's just my wife's physiology that makes it more likely for her.

Either way, seeing glimpses of her intimate parts in public places is now a event to expect rather than hope.

Actually the one piece suits with string ass can get quite loose around the pussy and ass, showing more than bikini pants which always fit more snug as they would else drop.

Never seen it with my wife but she has quite a big butt so normally you'd see her asshole only when she spreads her buttcheeks a bit. On the front everything is covered as well if she wears a thong bikini but she has an innie pussy, that might help. She has no labia sticking out, it's basically just a slit. Sometimes you can see a bit of a cameltoe through the fabric but never anything else. Everything that should be covered is covered.

Mine does too (big butt), which is what surprises me, her cheeks just seem to open up when she lies down. In a normal bikini you wouldn't notice but in a thong there's more rosebud than fabric a lot of the time. Perhaps because it's big and curvy but also quite toned from exercise.

Front slips are more of a rarity, it's normally if she's wearing it too loose. As she also has a very neat slit, it's mainly just smooth skin you see.

Different build, I guess. My wife's cheeks don't spread like that when she lies down like that. The thong is still hidden between her cheeks and I can't get a view of her asshole that way. But she has quite a fat ass and it's less toned, although she does go to the gym.

Disappearing thongs are just as good. Your wife's ass must like the taste of bikini more than mine :-)

Was it like my friend’s or less lip?

Wrong thong
 

Not quite a malfunction but similar.
On our last holiday, my wife packed an old and very conservative bikini, her current favoured cheeky / thong bikini and a risky almost G-string bikini.
On the first day she realised she had packed the two sizes of tops she has for her favoured bikini but not the bottoms, so she was left with the choice of the conservative or the risky.
Day 1 she wore the conservative but every other day it was the risky.
I would say it was fine for quiet beaches but not totally family suitable for the hotel pool, but that's what we got.
In the cheeky thong she would have blended in, but in her risky thong she looked near naked so totally stood out and got a lot of attention.

A big step
 

I was just thinking about something that happened a while back with a friend of mine during summer break. I can’t remember exactly where we were going—doesn’t really matter. She was wearing a denim maxi skirt with a front slit that came up just above her knees. Another friend was driving, and it was a full-size SUV, pretty high off the ground, so for a girl, it took some effort to climb in with a wide step.

Getting in was fine, but when we reached our destination and she was getting out, she misstepped and nearly fell. As she took a big lunge to catch herself on the ground, we all heard a loud ripping sound. Her skirt tore right up the slit, almost all the way to her crotch. I was standing nearby, helping my wife out, and got a clear view of her yellow panties. We all burst out laughing—apparently, that skirt wasn’t even cheap!

I've posted this before but I'll do it again. Before she was my wife we had gone to a Red Sox game at Fenway. She wore a white cotton top with no bra. We got caught in a massive downpour and the top went transparent. She didn't realize it until we got under the bleachers. But it was basically like she had nothing on. T