By far and away, losing the key to my old HISS-equipped VFR800 VTEC.

I'd bought the bike with a single key, and knew that was not a good situation to be in, so one of the first things I'd done was go to get a new key cut. Easy enough - but this was the first time I'd encountered an immobilizer-equipped radio key, so the new one was just a regular blank; it would turn the ignition barrel and open the seat lock perfectly well, but did not allow the engine to start. I'd begun to look into getting a new radio key, but it was a long and involved process requiring either a special dealer tool or a wire-splice hack that people on forums were describing with a not-guaranteed success rate; and I would need to get a new radio key blank to start with, these were not easily available locally.

So, at some point I lost the one working key I had. And if you know the Honda Integrated Security System, you know that in its most ridiculous early-2000s form, the bike would come from factory with two keys; and using either one of these, you could program two more; and that was it. It was integrated into the ECU, and without one of the two original keys (or if all four keys had been programmed), not even the Honda dealer or Honda factory could do anything about it.

The nuclear option was ordering a junkyard ECU from America, where the local-market Interceptor version of that bike never had HISS to start with, so it would ignore the fact that the radio receiver was not acknowledging the key. However, I got lucky, and found an eBay seller that specialized in HISS bullshit. They would go around UK junkyards, find totaled Honda bikes, pull their ECUs and existing keys, and use a donor bike in their workshop to program uncut blank radiokeys to that specific ECU. It cost me something like three hundred euros, but I had a replacement ECU and two blank radiokeys that, when placed near the ignition barrel, would allow my properly cut but non-radio copied key to start the engine!