It's possible I misunderstood what I was seeing, but it appeared from where I was standing that the Burma-Thailand border is fairly porous.

Walking across the bridge from Thailand to Burma, we saw two laughing young men climb under the fence just before the Thai border checkpoint and run off into town.

Then, on the way back to Thailand, I watched at least 15 people wade across the river, climb up onto shore on the Thai side, hide below the sea wall and converse with a vendor sitting atop it, and then climb up over the wall and into the adjacent market when they'd apparently been told no immigration officers were nearby.