Rarely, when a player holds or wears as armor, an item with special attributes on a server, those attributes can get stuck to the player.

Special attributes such as: Health +10, Damage 5, Speed -1 can all get stuck to the player if the right conditions occur.

According to a fellow server administrator, this issue can occur if a player logs in to a server while having logged out wearing or holding items with special attributes, and gets their inventory wiped by a plugin on login.

I can imagine this might happen because the server still thinks the user must be holding the item or wearing the armor maybe?

In any case, when this issue affects players, it massively unbalances pvp servers, and forces server admins to manually regenerate the incorrect NBT tags.