It's the air bubble in the cartrige/resevoir which is the problem. It will expand more than the ink and push the ink out through the nib. Two ways round this. Either ensure the resevoir is completely full or empty (no air means no expansion, no ink means no ink!) or store the pen inverted so that the nib is at the top. This means that when the air escapes, it will be at the closest point to the nib and no ink should be expelled.
Speaking from experience.