Oh dear. Found by CitrineFool and confirmed by me.
Stepped on a depletion block for the first time on the server and it instantly set my level to 30 prior to depleting it (i had used the enchanting table multiple times prior to this on the server to note).
On further inspection by expending levels using the enchanting table and then immediately jumping on the depletion block instantly increases your level by (roughly?) the number of levels spent in the enchanting table, prior to depleting it. Seemingly up to max 30. Duplicated the bug every time about 20 times in a row.
E.g. 1. Have some xp.
2. Spend xp in enchanting table.
3. Jump on depletion block
4. Have all the xp's.
Summary, there's a serious exploit with the Depletion Block.