Whenever I am messing around with the slime block boat bug, I got a message after flying a few hundred blocks in the air, saying "Flying is not enabled on this server," although I was in singleplayer. I was using a birch boat, the Elytra, and a 7x7 square of slime blocks.
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is duplicated by 5
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In 15w47b! This needs to be fixed!
is this still an issue in 16w03a?

This cannot be reproduced anymore due to the fix of MC-90352
@unknown you can always teleport yourself to huge y coordinates
Ticket resolved as incomplete, because no answer in a reasonable amount of time (1+ year), if it still happens, please update the ticket.

As far as I can remember this was changed some versions ago. Now in 1.11.2 flying is enable for singleplayer (net.minecraft.server.integrated.IntegratedServer.init()
).
Relates to:
MC-90352
Based on MC-90882 this also happens with tnt player launchers