The part of standup identifying blockers is to figure out when someone doesn't realize their blocked and/or they're spinning their tires a bit. If someone is blocked they should reach out immediately.

Instead, standups provide the team the ability to identify roadblocks the developer might not realize. Perhaps you're working on something and I've been down that path before, and know your solution won't work for reasons you're not aware of. I can tell you that now and save you several hours of banging your head against the wall.

Has this ever actually happened though?

Frequently, especially with junior developers.