A player with the slow falling effect that lands on a "bouncy" block, like slime blocks, or a bed, will start jittering up and down around 0.035 blocks. This state persists until the player steps onto a non-bouncy block.
To Recreate:
Give yourself slow-falling
Jump or fall from any height onto a slime block or bed
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We're tracking this issue in MC-151706, so this ticket is being resolved and linked as a duplicate.
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