For once the "Never should have come here!" line that enemies like to use would have been appropriate...
I had spent the night at Old Hroldan, and in the morning decided to head up a path that led into the hills. I knew it was risky, since the Reach was enemy territory (I'm in the middle of the civil war, currently liberating the Reach; all Imps, Thalmor etc. are instantly hostile [Civil War Overhaul feature]) - but I did not expect this:
A Forest Dragon decided to attack just as I was heading back down to Old Hroldan (I didn't want to fight a guard).
I eventually Fus Ro Dah'd the Reach Hold Guard, and the dragon killed him.
Later during the battle the dragon decided to pull a nice (I mean it) little Limit Break (Dragon Combat Overhaul feature), and turned semi-invisible:
Some Forsworn, vampire's thralls, bandits and animals all took part in the battle. I have to say, it was a fun fight.
Later during the Reach campaign (after I finally found some Imperials and got myself disguised):
Finding the evidence was a huge pain, and I had to resort to using Muffle (that I wouldn't have been able to cast if Ondolemar, who somehow saw through my disguise and attacked me, hadn't had a Ring of Major Illusion).
So, as I was leaving the city (the guards had just killed Miraak's cultists for me, by the way), I basically said "Screw you, Markarth!" and changed back to my real armor. That of course caused the entire city guard to attack me.