It just happened in my game. I made a screenshot, but I'm having difficulties uploading it. I'll do it when it's done.
In the meantime, discuss ?

No it is normal depending on Ur main quest and civil war choices.

I know that Imperials take over Riften if you sided with the Imperials. HOWEVER, I've joined the Stormcloaks and am up until Message to Whiterun. There were THREE stormcloak soldiers inside riften discussing why they joined the Stormcloak.

I have the game since its release and have NEVER experienced this.

The rest of the guards were all Riften guards though. (purple clothes).

Weird!

It's not 1.9.

I have seen groups of three Imperial soldiers in Whiterun and now just recently in Solitude. They have always disappeared soon enough and haven't come back. I suspect it's because of a mod - if you could provide more information on your setup, it'd be great for us both.

I have ALL of the DLC's, yet I do not have any mods installed. My game is completely "OFFICIAL".