The thread I found this on :
http://www.neogaf.com/forum/showthread.php?t=1346789
"The polygon preview said they had hardware problem with the controllers becoming unsynched. That's not good." polygon
"Same problem with mine. I think it is only one of the two Joy-Cons specifically however -- the left one. We've reached out to NOA and will update." -not sure who.
"During one part we do bring up two technical issues I have with the Switch, which I'm really hoping are just isolated to our console. One is with the battery not charging all the way even when it's docked, and the other with some issues of input lag in rare cases when the Joy-Con are detached from the console itself.
Edit: Oh! I see other sites have had issues with the left Joy-Con as well." Gamezone
Digitalchumps had this issue too.
this guy had it:
Nintendo World Report:
"Yes - when off the system. It went away when I attached it back to the Switch. Still worrisome."
"It happened to me both on the Grip and separately. No clue as to cause."
The Boat : "I can confirm this, my left Joy-Con's stick also acts up randomly. Sometimes it stays ages without happening, bit it usually comes back. It's not very regular, but it's very annoying. The game doesn't pause to resync, because the controller is still synced. It usually goes away in a few seconds."
Hopefully its just something that needs a firmware update or something to fix it.
If its a hardware issue though, thats gonna suck for those that pre-ordered.
