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 :-)

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

Many come to mind
 

One of the best..although I wasnt there. Was from an ex. She told me and I later asked some of the people there and got the whole scoop.
I dated a little Indian girl for awhile, thats feather not dot. She was gorgeous. Short spunky and meaner than hell when she drank. I cant remember how it came up but she told me a story about a party she was at just out of high-school. She'd been about 20. Anyway she was back home from college and her best friends younger brother threw a party. She was all dolled up in a little black skirt and ended up passing out leaning back against the couch with her butt on the floor. She said when she woke up her skirt was flipped up and ,because of the way she was sitting, her underwear had wedged up in the front and her vagina was showing. She was mortified and furious that Noone fixed it for her. I loved every bit of it..for a little thing she had fat pussy lips..she hated them I loved them. She went on and on about "just knowing who did it" calling her all kinds of names.
Later on in life, long after we had broken up, she was honestly just to much of a mean b*tch, I ran into a girl that was there. According to her my ex had gotten drunk started her bullshit with a group of younger (18yr olds.seniors) and was especially picking on some chubby girl..not surprising to me I watched her do it all the time. Anyway once she passed out and the party had died down to about 10 people (all very drunk) the chubby girl went "hey watch this!" And without warning flipped my exs skirt up and the place went wild and because of how much of a bully my ex was Noone helped. She said it was the funniest thing she ever saw. The meanest b*tch around getting totally embarrassed by a high-schooler. And to make matters worse my ex had wore a tiny pair of undies and they basically disappeared up her crotch, giving her the biggest front wedgie she'd ever seen. Both lips completely out. The guys were speachless and the girls cried with laughter. pictures were taken and yes I had one at was able to get ahold of one but lost it with the phone. Still the thought of my gorgeous Stuck-up ex getting served one of the worst wardrobe malfunctions of all time by someone she was picking on and then the younger boys she snubbed ending up with pictures of her big phat pussy to look at whenever is like poetry

Less. There's really just a slit visible.

Pussy peak
 

My wife also has an 'innie'. It's also low down so she looks quite Barbie-like. So, similar to you Martin, when the thong is in place you can't see anything but what happens is that, if it's a bit loose, you can see through, sometimes from one side to the other, if she twists, bends forwards etc.

It's only a momentary thing that you'd need to be watching to see, but it seems to happen often. If she had any pubic hair it would probably be much more noticeable :-)