This appears to be a conflict between signshop and any vanilla mod that creates items with non-vanilla NBT data on them. So far I've been able to reproduce the issue with power suit mod. If you have modules attached to the powersuit in your inventory but not equipped then buy an item from a signshop, it will wipe the nbt data and thus remove the modules from the item. There have been mysterious nbt errors before that I did not know what to attribute it to but I suspect that this may be related as well.

I'm having a look myself through the code to see if I can find the issue, but I assume you are much more familiar with the code so I hope you may find it more reliably than I.

posted on bukkit dev