When editing a mob's NBT tags, it is possible to give it custom items using the Equipment and DropChances tags. This can be used to give bosses custom drops, among other things.

However, there can not be more than five such custom items, and thus the mob can be given, at most, full armor and a held item. If a map maker wanted to have more drops, they must sacrifice an armor slot or make the custom drop a held item.

Attempting to add a sixth (or more) custom item will cause the game to crash.

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Sorry if this is not considered a bug, as the crash is only caused by external editing. Will there ever be an "Improvement" or "Annoyance" category here? It's a shame if Mojang's abandoning the wiki's buglist; how will we report annoyances?