In pocketmine-mp before 4.18.0 a "mismatch" type InventoryTransactionPacket is sent by the client to request a resync of all currently open inventories. Since PocketMine-MP does not rate-limit these "mismatch" transactions, and the syncing of inventories is not deferred until, e.g. the end of the current tick, they can be used as a very cheap bandwidth multiplier by making the server send out many MB of data (network serialized inventory items can be very large, especially when dealing with large amounts of NBT). This is not currently known to have been exploited in the wild.