Bizzare, inexplicable, yet reproducible math errors

Please take a look at this and tell me if this makes *any* sense to you.

I have the following code:

Code (csharp):

1.  
2.                 float delta = DesiredHoverHeight - GroundDistanceAvg;
3.                 float upwardDirection = GetUpwardDirection();
4.                 Vector3 newPost = new Vector3( 0f, delta * upwardDirection );
5.  

For the sake of this exercise, upwardDirection returns 1. Delta is some number between 0.25 and 0.75

In this case, 0.270584

I would expect to get the result:
Vector newPost = { 0.0, 0.270584, 0.0 }

Here's a snapshot from the debugger:



WTF? How on earth does newPost return 0, 502, 0??

I have this at 100% reproducibility - which doesn't even make sense if its a memory corruption issue - and it works at other times, indicating it's not a code issue....

I am so confused.