I made a slimeblock tower on a sticky piston connected to a clock. When I fly up under the block I fly through.
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Invalid ticket. You're in spectator mode:
/gamemode 3

It could also be because the blocks disappear shortly when pushed by a piston, letting a player through.

That's the opposite of MC-1230

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.

Ticket resolved as incomplete, because no answer in a reasonable amount of time (1+ year), if it still happens, please update the ticket.
Are you in spectator mode? That could be the issue.
Also, please don't put down the affected version as a future version! You DON'T have access to it yet!