Not sure if this qualifies as a bug *as such*, for all I know the AI is just way, WAY too good at slingshotting tech....
But if various religions get established far too early, and Liberalism is discovered around the BC/AD changeover, that kinda ruins my immersion.
Playing as Egypt, normal game speed, Viceroy, SVN 615.
Persia discovered Liberalism in turn 141 (noted while playing)
Now, various religions had already popped up earlier than expected, but I didn't think too much of it at that time, but when Liberalism ALSO appeared, I got worried, so I checked the history playthrough when I completed my UHV (With a bazillion Roman barbarians standing ready to burn my cities..)
535 BC -Catholicism in Persian Jerusalem
520 BC -Islam, Persian Babylon
475 BC- Protestantism (it didn't mention where, though it did say Persia created the all saints church)
Now, it MAY be that all the history I've been told is mildly off, but this seems a bit too much.