Just a quick note on that. In this scenario I had to target the planet I was stuck in (The closest body) as I was still in the planet in no-horizon mode, Then fly away )Black circle worked as a guide to which direction to fly. Then got to a distance I knew would put me outside the planet. Re-Logged then found myself 400km above the surface and no-longer mass locked
Here's my edit of the day. Including my brush with near death due to being in the wrong frame of reference. Several commanders lost their ship this way and had to race back.
Then a shot from the SRV as the bodies collide. Just at the start of the video you can see the SRV starts falling through - a common theme when the bodies were close, unfortunately. Around 3:00 was my first attempt to swap from c to b - it looked like it might've worked until I got hit by some invisible force. And soon after, a planet - whereupon the lighting went ghostly.
It took a significant effort to escape once the planet had closed up on me - I reloaded in non-Horizons mode (where the planets were not colliding, interestingly) and it put me local to the c body, as that was where I quit Horizons... but because the bodies were close, I was actually only a few hundred km from b - and mass locked to it. Reloading in Horizons saw me back on the surface of c again, because I'd not been able to jump or otherwise force a "save" to happen. Back to non-Horizons, and a lot of faff later, I managed to eventually jump away.
Next up, I thought I'd try and land closer to the collision (from b this time) - but my ship also started to fall through the floor again... and well, things didn't go so well...
My last attempt, I tried the same again but from an area that wasn't being eaten quite as quickly. I managed to land, deploy the SRV and dismiss my ship (just in time, it turns out). I tried another go at switching bodies but ended up inside the terrain again
I've had that happen even in "normal" bodies. It's like a mismatch between the body's "collision box" and it's visible shape. The small pebbles get placed on the actual surface, so you can use those to judge the actual shape of the terrain.
On the subject of 'That's not scientifically possible' ... in theory the orbit sharing actually is, but the moons themselves wouldn't touch, instead the gravitational interaction would slow the inner moon down while speeding up the other so that they swap orbits...