I don't think you've run into over 100 mines before, because I think there is a limit on mines in a sector. I went to a purely defensive game just to try some things out the other night, and went mine-crazy. Just kept spitting out mines as fast as I could. Once I had 100+ mines over a particular planet, placing any more resulted in an error that said that there could only be 100 mines. However... If I place 99 mines in orbit, and then turned around and put more than 1 in...the mines would go into orbit, BUT subsequent attempts and putting up more mines failed with the Only 100 mines error (even thought I might now have 130 or so mines in orbit already!
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Codo