When a supply depot is built near the edge of the building area, and supply is deposited into it, that additional supply will not show when viewing the supply on map. The supply is still usable while in base, but i'm pretty sure that this "invisible" supply can't be used to spawn in. This discrepancy on visible and real supply, can be very annoying for a variety of reasons. Also, in HQC mode, when you have the task of resupply base, this doesn't allow its completion.