Item stack swapping can destroy/duplicate items during severe lag.

[Nebulon-B_Frigate_FTW]

While undergoing severe lag (up to 5000 ms ping time) recently I tried to swap white hardened hull blocks with whit lights and the lights didn't take the place of the hull. Further movement (I thought it was a display bug and in Minecraft further stack movement usually updates things) of the hull blocks created duplicate stacks that could then be combined with each other and the result was around a tenfold increase in the amount of white hardened hull blocks I had. Additionally, it happened again during a lag spike while I was swapping black hull with power blocks. Black hull was doubled and all the power blocks were lost.