Alright, this has happened three times now with the same save. The third one is still ongoing as we speak.
Every now and then I've been getting an absolutely insane quantity of golden cookies spawning. I play on chrome and use the uncanny cookie clicker extension, which autoclicks all of these for me, but no other cheats or add-ons. The first time this happened, it took my prestige from +1 to +456 on a run where I had not yet ascended. The second time, still with no ascensions in the same run, it took me to +73 million. In the current one, still ongoing as I type this, is closing in on +1 quadrillion after 2 ascensions. One person suggested it was the extension playing catch-up when I switched tabs, but the second time it started while CC was my active tab and the third I realized it was happening when I noticed the tab header said 'Infinity' rather than a quantity of cookies. Each successive incident also seems to go on for longer.
Below are screencaps, the first is from the second incident and shows it with the golden cookie autoclicker active, second it the third incident when I turned off the cookie autoclicker. If it's useful to anyone I have exported saves from during or shortly after all three incidents.
Only other thing I can think to mention is that this does NOT give the four-leaf cookie shadow achievement.