A new screen in our operations console appeared inside two identical frames. Two sidebars, two headers, one page. The founder spotted it in thirty seconds.
The cause was a registry we already had. Screens that draw their own frame have to be listed in one file so the layout knows not to wrap them again. The new screen was built by copying an existing one that draws its own frame, but the registry line that goes with that pattern did not come along for the copy.
The fix was one line. The lesson is older than software: a pattern is the code plus the paperwork that makes the code safe, and copying half of it is worse than copying none, because it looks complete. We have a note about this exact trap in our engineering guidance. We walked into it anyway. So the note now names the file and the line.
Why write this up at all? Because the console is where the founder runs the company from, and a screen with two frames is a small tax on every visit until someone notices. The person who noticed was the person paying it. We would rather the registry noticed, so the next step is a check that refuses the push when a self-framing screen is missing its line. The trap gets a tripwire, not another note.