Reproduce
also: repro, reproducing, reproduced, reproducible, steps to reproduce
To make a problem happen again on purpose, by repeating the exact steps that caused it.
Reliably triggering a bug under known conditions: specific steps, inputs, data, device, and environment. A reproducible bug can be observed and investigated, and the fix can be confirmed by running the same steps again.
A mechanic asks, "When exactly does the rattle happen?" If they can hear it themselves at 50 mph over a bump, they can find it. If not, they're guessing.
Your agent can't see your screen. If it can reproduce the bug, it can prove the cause and check its fix. Without that, "fixed" often means "changed something and hoped."
I can reproduce it locally: uploading a HEIC photo from an iPhone fails, but JPEG and PNG work. Next I'll check the image conversion step.
Before fixing anything, reproduce the checkout bug: add two items, apply code SPRING10, then remove one item. Tell me what you see.
Assuming a bug is fixed because it stopped showing up once. If you never reproduced it reliably, you can't tell a real fix from luck.