My Next.js 16 button was visible and completely dead in production. Here's why.

I added a tiny test page to confirm my error monitoring was capturing frontend crashes. A title, a paragraph, one red button that throws an error on click. The kind of code you'd send to a code review and apologize for being trivial.

Locally: worked perfectly. Click, error, captured, done.

In production: the button rendered. Click did nothing. No error in the console. No network request. No visible feedback. Just dead HTML that looked exactly like a button.

I'm a solo dev building an iOS market intelligence tool, and this bug burned an hour of my pre-launch sprint. The cause is a real Next.js 16 trap. The fix is well-documented. The path between "this should work" and "ah, it's this" is what I want to write about — because the same shape of bug is going to bite a lot of people once Next 16 spreads.

What I had