Ed Kelly

Can confirm in 14w21b. I was showing a friend a duplication glitch, and we both lagged out. (The item frame one, totally insane lag spike)
Anyway, when he logged back in there was a second him, which caused the GUID to appear on the scoreboard...
I killed the clone, and it dropped copies of his items and experience. The scoreboard keeps track of rather important things on this small 9 player server, so I had to manually fix the scoreboard.dat in NBTExplorer.

I encountered this, and didn't even realize that it was an error...
After seeing which blocks it happens on, I think I know what's going on. The new F3 screen includes the various model names, and variation names. This information is evidence that the devs are making a transition toward moving the game away from a purely numeric world-save structure. As a part of this, the variations and models are no longer handled by adding 'damage' nibbles to the end of the block's data. However, as part of the transition, the code which adds those nibbles still exists. I'd venture that this bug will simply disappear on newly placed blocks after a certain version, but I don't know that it will fix for old blocks placed prior to that version unless they add code to retroactively fix blocks.
EDIT:Also, confirmed for 14w21b

I know that I created a report as well, but to be honest I see more potential in this bug than issue. Careful building will prevent fall deaths, and similarly careful building will enable completely hidden entrances...

Edit: Also, I'm 90% sure that the bug is caused by the fact that the player's lower body is clipped into the ground when in a minecart, but the player's body isn't responsible for the motion while in the minecart. So, when you break it you continue clipping instead of being forced into your correct position as you would when you exit the vehicle proper... But, that's just a likely cause. 😛

I made one of the issues marked as duplicate, and would like to add that it also works while minecarts are ON the rail as well.

This is the intended behavior, it's called repeater locking.

I think that this has to do with the new "Attribute System" that the snapshot post on mojang.com mentions.

My server spit out this message when I tried to sprint the first time on 21a:

2013-05-26 16:55:45 [WARNING] Ignoring unknown attribute 'Speed'

And... I just realized that this issue is resolved already. :/

Chili Fries is correct, the issue is related to the scoreboard, in particular the health objective type. After the snapshots not working at all for me due to this issue, I removed my health objective and upgraded the server; alas, it works fine now.