I made a visual explanation of what's happening:
I'm having a problem using the Skin modifier's Mirror Mode in Max 8, and thought I'd ask about it here in case it's a known issue. When I attempt to Paste Blue to Green Verts, the Green Verts half will then be messed up. To be exact, everything I gave a weight of 1.0 to on the left half will then also take the flipped verts on the other half as weighted 1.0 to the left half, as the visual explanation shows.
I found the minimum threshold number that doesn't have any red dots in the mirror, only blue and green, so the threshold should be fine.
Also, if there's a third party kind of program that also mirrors weights from one half to the other, please let me know about it and I'll use that instead.