Hard to tell, i have stock price of ~11k now (my tactic seems to work LOL), others ranging from ~12 bucks -> 284 bucks. For reference a supermarket build price is around 1,030.000. Got an anual profit of around 1,5 bill (year 2016, started in 2000) using the 10 city script from
real-world 6.60 which i slightly modified to my liking so its easy mode. I may try again on the stock settings now i see my strategy works like a charm

It happens randomly, but i noticed it's probarly when a company does a huge share dillution, like 50-55% so i lose control at the exact moment. (i own every company for 100% and try to keep it like that, making to much money with real estate lol) it happens if that message pops up at the same time as another message (like a pay raise for my CTOO or something)
If you want i can zip the savegame and script so you could try it out yourself, but i don't think its due to the script. Happens with the script and without the script
Greetings
Aadje93