The bug

Attribute Modifiers not being removed correctly.

How to reproduce

1. Go to y=150
2. Place a block under you using:

   /setblock ~ ~-1 ~ stone

3. Get an item that applies an extremely high speed boost. For example:

   /give @p golden_boots 1 0 {AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"speed",Amount:100,Operation:0,UUIDLeast:734432,UUIDMost:4374733,Slot:"feet"}]}

4. Equip the item
5. Stand on the block and hold down the move forward key for about 5 seconds.
6. Remove the item. The speed boost will remain.