When a player stands on top of a honey block being pushed or pulled by a piston, it moves the player with it. But if there's a lag spike at the same time, the block will move without moving the player, causing the player to fall off the edge of the block even though he should be stuck to it.
This is quite noticeable when using honey blocks as a "seat" in flying machines over more than a few blocks, where any lag can cause you to fall straight off the machine to your death.
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@Mikeware I've only tried on a single-player, normal terrain world, but my computer isn't great and lag is common. I can only imagine it's worse for people with even worse computers than I have. I've been able to mitigate the effect a little by surrounding the honey block with slime blocks, as they tend to push the player or mob back into place when there's lag, though it's not 100% effective and obviously is just a cumbersome workaround.
Client movement is known to not work well with server controlled things like flying machines.
A workaround is to sit inside of a Minecart placed on honey. Since minecarts are calculated server side, the issue does not happen in that case.
Thanks @NeunEinser, but I would hope the new functionality of the honey block would have removed the need for a Minecart in a flying machine, as they can be bumped off the flying machine by the player. Since one of the features of the honey block is that entities stick to them, it'd be nice to see this work still with a spike.
I've seen this occur without a server involved too just on local single player too, so there is some sort of desync happening somewhere.
@Mikeware it's still client/server desync. In single-player worlds, there's still a server, it's just an integrated server. But it handles all the same things a dedicated server would.
I can confirm I've experienced this in 19w42a as well. If you use amplified terrain generation to peg the GPU, you'll exacerbate the issue.
You can see this occur on the stream I did last night here just after 3:09:36: https://www.twitch.tv/videos/495712598?t=03h09m36s
Everything is fine until about 3:09:51 when everything freezes for a second and the flying machine I'm on pauses. When the lag spike ends, the machine goes 'double-time' to catch-up to where it should be, and I'm left behind. I'd expect the accelerated updates from the lag to still move me along as I would have been when no lag had occurred.
Seemed to happen to me on 1 out of every 3-4 trips or so, but I didn't notice it as much in my original flat testing world, so definitely a CPU/GPU bound sort of issue with the lag, which was more prevalent here in amplified, but seems like it'd be a good indicator that this problem could arise in multi-player as well. In my case I was just playing single player on my own machine.