I was just there a couple months back. We were at one of the larger pools, I think at Grand Prismatic Spring...if you've ever been, you'll remember it's where everyone seems to lose their hats since you seem them all over the place. Idiots.
The family in front of us on the boardwalk was dawdling along with the mom in front and the kids trailing behind. One of the kids thought it'd be fun to balance and walk along the edge of the boardwalk in a spot where there wasn't a railing... so I doubt the ground below was some of that fragile crust you could fall through, but there was still that few inches of water flowing by and the lovely bacterial mat that grows everywhere.
As you'd guess, the girl slipped and started to go in until I snatched her arm and hauled her back up. Dear mother had no idea what just happened even as everyone around was gasping at the whole ordeal they just witnessed. Girl ran up to mom crying and I still doubt she ever knew her 6-7 year old kid nearly fell in.
Suffice to say, there are a LOT of visitors to Yellowstone, there are signs literally everywhere, but you simply can't fix or prevent stupid.
It’s impossible to make something foolproof because fools are so ingenious.