You know, it's been a long time, but I have seen epoxies that are not to be mixed at 50/50. If you're using a 50/50 and you have the ratios right and mixed properly, it's unlikely to cause you these difficulties.

When I mix epoxy I usually use a paper plate. One thing that I have noticed is that my mixing is sometimes lacking, so I try to avoid applying the epoxy from the edges of my mix. I've found that if I'm going to make a mistake in mixing, the problem is always around the edges of my epoxy "puddle."