In the beginning of the movie when the boys are at the scrapyard buying the car there are leaves on the trees so it's probably late summer. Next shot is them driving up north with no leaves on trees (late Autumn) and when they cross the border there's already snow on the ground. Did they just spent 3-4 months driving up north?
Boat seen at the Vladimir village in Russia clearly has quite a modern western (or Japanese) outboard motor on it. Not available (or affordable) in a remote Soviet village of that time.
The Russian army seems to only use GMC and Volvo trucks.
How did the boys just happened to get a shotgun? The gun laws in Finland are pretty strict and you need a buying permit from the police even for a hunting weapon like a shotgun, something a foreign tourist would never get. It's also highly unlikely they would happen to get one from the (non existent) black market just like that.
All the Russian uniforms are wrong. They wear typical Finnish field gray uniforms.
The book KC is reading in the car is a Finnish road map guide (written in Finnish), not a tourist guide.
None of the buildings or people in the film have a Russian look, which is natural as it was filmed completely in Finland. The only Soviet things you see (besides a few AK's) are the two cars right in the end: A Lada and a Volga.
Why does KC shoot the priest first and not the guy pointing the shotgun? Is a priest performing a sermon really the first thing to take out in situation like that?