According to the shows TV Tropes page, the Berlin incident everyone keeps mentioning relates to East German forces trying to follow a couple defecting to West Germany, killing them and almost provoking nuclear war. However, I’m having a hard time finding any confirmation of this.
That said, there’s apparently an iOS app out called the For All Mankind: Time Capsule which is supposed to contain more lore, so it’s possible it’s squirreled away in there
Yeah, for like half a day if you went on the Apple+ page for the show, there were eight news reports for things that happened between season 1 and 2, including Berlin. When I checked last night, they were already taken down for some reason (I guess so you’d download the app).
East German guards pursue a couple like a block into West Berlin and kill them in the street. US troops exchange fire in return. The whole city goes on lockdown, which cascades to DEAFCON 3. The resolution is yadda yadda’d, I think, but whatever happens, things calm down after a bit.