If you are in survival and you hold left click to place scaffolding then at the top when you have no more scaffolding it will have place ghost scaffolding that you can climb but if you break it won't drop anything
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We're actually already tracking this issue in MC-148795, so I resolved and linked this ticket as a duplicate.
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Behavior can be reproduced in 1.15 Pre-Release 2 by repeatedly toggling between breaking and building the scaffolding, no matter it being in survival mode or creative.