Normally, mobs get pushed by solid blocks to a nearby air space. But cave spiders can actually climb out of a place where they are suffocating if there is an airspace diagonally above them.

To replicate, get a bunch of cave spiders inside a 1x1 space. They will usually try to climb to the top. Next, push a block into them with a sticky piston. There must be an air space above the piston's arm. Very rarely, a few of them can glitch through the block by climbing out. Now, retract the piston before they die. Now, several of them will glitch out. They should not be able to climb out, but their climbing behavior allows them to climb inside of blocks to get out.