Experience levels changed suddenly, or are inaccurate

[Icohedron]

Experience levels may change unpredictably or may be entirely inaccurate due to these bugs:

Player levels may jump up by 1 level after using '/bottle 0'
SpongePowered/SpongeVanilla#340

The experience lost after enchanting or using an anvil can be regained after using the command '/bottle 0'
SpongePowered/Sponge#1322

The problem is, as stated, that the Minecraft server tracks total experience and levels (and the current fraction of experience between two levels) separately. When changing a player's total experience via the /xp command, Minecraft also ensures that the experience levels are updated. However, this does not happen the other way around, if the levels are updated, the xp is not. This doesn't just happen using the /xp command, you also observe this behaviour when you repair an item using an anvil and enchant an item using the enchantment table. -- dualsprial