I had a flash ad take 100% of my cpu back around 2005 or so. It wasn’t even trying to be malicious, just a poorly made ad. That was the day I stopped allowing any site exceptions in my ad blocker.
Of course 100% of that cpu is probably 1/10 of one core on any of my modern machines, so an ordinary and not-broken ad laden page routinely eats several times as many cycles now. Progress!
Haha, or because it froze the whole browser for a few seconds upon loading. Unlike Macromedia Flash by the way.