Tab complete on the second arg in large packs will cause the client to lock up and the player to lose connection.

[Claycorp]

Look into this issue further, Tab complete might need to be updated.

[Claycorp]

After further investigation the reason behind this is because you can ban items without holding them. The second arg can be one of tens of thousands of items in some mod packs thus tab completing it will attempt to return all of those options at once. This can't be avoided AFAIK otherwise all tab complete for partial arg two entries would be lost.