Had the lawnmower suddenly die on me out of nowhere today. Couldn't get it to start back up even with starter fluid, replaced the spark plug since it's been a while, still nothing. Spent a good chunk of time troubleshooting, narrowing it down to an ignition problem, and tracing out the wiring on it and eventually found that the safety interlock module failed and was keeping the magneto's kill wire grounded.
Fortunately to finish up today I could just unplug the damn thing and it would run again, albeit with all safeties disabled. Gotta get a replacement for that before too long though...