Well in the Companions quests he came with me whether I wanted to or not, and didn't count as a follower so I couldn't even tell him to wait further back so I could sneak forward. The second time, with the centurion, the centurion found me even when i was sneaking so I had to fight it anyway.

quite early on in the game, when Lydia was still alive (sigh), I was crawling back from a dungeon fully laden. Made it all the way back to Whiterun, was outside the city when 2 Thalmor attacked me (they attack me on sight now since a little misunderstanding I had with them after a failed pickpocket lol). These 2 were taking me dowm, managed to kill one but I had no potions left and Lydia was nowhere in sight! Was just about to lie down and die, when the Thalmor takes and arrow to the face and Lydia pops up from behind a rock! That was a bit of a wow moment.

I think the most awesome wasn't a follower at all, it happened on my first play through, on the way up to High Hrothgar, met that Ice Troll, Lydia died to it, so I ran off, occasionally checking behind me to see if he was still giving chase, he chased me all the way into HH, then the greybeards destroyed him, wasn't expecting that a Troll could use doors, and I also wasn't expecting the greybeards to slaughter him within 5 seconds of him being present.

I had J'Zargo with me and we met some bandits. They come near while I charge my dual cast chain lightning. Just before I release all my wrath on the bandits J'Zargo charges past me to give the bandits a beating...I fire the chainlightning and see my good ol' friend being pushed far far away. In fact I never found his body...and it was in a vast and open plain... . I had to laugh so hard at this. Motivated follower charges and a second later the sheer force of my lightning sends him I-don't-know-where. He was NOT disintegrated. I double-checked.