>Plums are dangerously poisonous
...What? All stone fruits seeds/kernels/whatever are toxic to SOME degree (well, I mean, there's few things that WON'T kill you if you ingest enough of it), but... I'm not aware of plums that have dangerous levels where you can't just eat them (unless it's some east asian variation).
The same can't be said for cherries. The cherry pit is safe to swallow intact, it won't harm you. However, there are cyanide compounds in the pit that will be rather harmful if exposed to air. So... don't eat crushed/cracked/split cherry pits. Thankfully the human liver is good at dealing with small concentrations of naturally occurring cyanide, so it's unlikely to be lethal, but... Don't chance it.