The router was working. That is the part I keep coming back to. It routed packets, it served DHCP, it never fell over, and in eighteen months I had touched it exactly once, to change the wifi password. So naturally, on a quiet Sunday, I decided it would be better with OpenWrt on it.
It would not. It would be the same, just mine. But the stock firmware was a bit dated and the urge to tinker is a fault I have never managed to engineer out of myself, so down it came off the shelf and onto the bench.
The flash itself went fine right up until it didn't. The web uploader took the image, sat at a progress bar, and then the device simply stopped answering. No lights doing anything interesting, no ping, no recovery web page on the usual address. I had managed to brick a router that, an hour earlier, had no problems at all. The classic upgrade: from "works" to "paperweight" in one click.
The fix was the bit I should have set up before I started. I opened the case, found the four-pin UART header the manufacturer had thoughtfully left unpopulated, soldered a header on, and wired in a 3.3V USB-to-serial adapter. With a console at 115200 baud I could finally see the bootloader, interrupt it, and TFTP a known-good image straight into flash. Ten minutes of soldering bought me the one thing the web interface never offered, which is somewhere to stand when everything above the bootloader has gone.
It is back on the shelf now, running OpenWrt, doing precisely what it did before. I learned nothing about networking and a small, useful amount about always having a serial console before you flash anything you cannot easily replace. Next quiet Sunday I will try very hard to leave it alone.