It could be the old and kind of tricky to get Chunk Resets which, from what I remember, use to reset the chunk if an invalid block was created/loaded. This is most likely fixed but it could be a possible explanation.
Added video of the Banner, Sign, and a Fence as a control.
Also happens for comparators
I was unable to get a 14w02c Server running so I tried it with it open to LAN and it still glitched the gamemode changeing
SinglePlayer world so far, I do not have a very powerful computer so doing a multiplayer world as well as a client would cause lag and to me not be accurate.
I believe I narrowed the problem down to the command block having the command "gamemode 0 @a", but when it's changed to "gamemode 0 {username}"it seems to work.
Tested on copy of world and after the first use it does not let the blocks break.
Cannot reproduce either.