I write simple code:
FVector Result = FVector::ForwardVector.RotateAngleAxis(90.0, FVector::UpVector);
And expect what result will be
{X=-1.0 Y=0.0 Z=0.0 }
But in fact i get
{X=-1.19209290e-07 Y=0.999999881 Z=0.000000000 } (about 45 degrees)
I try to reproduce it in Wolfram Alpha and get correct result
What i do wrong?
One of the answers: Speak in English 
Thanks!
Well, the forward vector can be anything because it depends on the orientation of your actor. Your idea is correct, you will rotate it as you believe it should, but you lack information to find out what is actually happening. Log what it is before you rotate it, what the length is before you rotate it and the same for after. Then you can compare to see exactly what changes. If you cant figure it out, post these values as an edit in your post or at least as a comment. Also, I wont see it again unless you comment to me so do that anyways.