I've never seen it. Once, I saw a Thalmor patrol and Stormcloaks fighting, but never Imperials and Stormcloaks, even though I've seen plenty of Legion/Stormcloak patrols. According to the UESP wiki, one of the random wilderness encounters is three Imperials and three Stormcloaks fighting, but not once have I ever seen this. It seems like it should be much more common, given, you know, there's supposed to be a war going on.

Yeah, they are a really rare occurance if it exists. Besides, the very guys who help you, can they report you for murder? I've had Stormcloak prisoners do that to me when I freed them.

I've encountered it once, but it was interrupted by a dragon.

I once fought off a dragon from the Imperial Rift Camp, but apparently I struck an Imperial soldier in the chaos. When the dragon died, every single soldier in the camp turned hostile, despite the fact I had just saved their lives. Epic.

I think there is one random encounter with three Imperials fighting three Stormcloaks outside of doing the Civil War quests. That war is really tearing Skyrim apart

I have found a few times, Stormcloaks and imperials fighting in the farm south west from riften.
One also finds Thalmor fighting Stormcloaks too.
I tend to provoke them in conversation if they are not already attacking.

It is surprising the Knights of the Nine had not grown since the Oblivion crisis.

Yeah, on the road behind the Riverwood Trader on the way to Falkreath. I believe it's a spawn point. I also saved some damned Stormcloak rebels from some Thalmor too even though I was pro-Imperial with that character.

I've seen Imperials fighting Stormcloaks at Darkwater Crossing.

Imperials fighting Thalmor sure, but Stormcloaks?
Don't be absurd!

I saw it once, in my first play-through of the game. It wasn't as cool as the time I found a bunch of bandits wearing Imperial armor.

Well, unless it's a product of the mods I use (although I don't think so) there is a copy of the Knights of the Nine book (a skillbook actually), found on an icecap.

But it doesn't mention the Order, been restored after the Oblivion Crisis, or the Crusader's Armor, having been used to defeat Umaril so, it must had been written before the events.

No, that's in the default game. Near the Tower stone, IIRC. There a flag there that marks the spot.

Yes, I've run across both Stormcloaks and Imperials, and Stormcloaks and Thalmor fighting. I've also seen both dead Imperials and dead Stormcloaks with the survivors of the patrols walking away. The fight must have happened right before I reached them.

Yes and the stormcloaks whoop them silly. Probably because the imperials didnt use their shields for some odd reason.