Left Joy-Con in particular appears to have reliability issues on unpatched Nintendo Switch consoles.
With just one week to launch, a number of people in the gaming press who have received the Nintendo Switch early are reporting connection issues for the Joy-Con controller, specifically the left controller.
The video below showcases the issue, which appears to be occurring when the Joy-Con controller is covered or something is between the Joy-Con and the console. However, that's also when intentionally attempting to replicate the issue, so it's not immediately clear if that is the only time it is occurring.
You can check out some other reports of the issue below.
I'm having issues with the Switch's left Joy-Con dropping sync. I've had to play it almost exclusively in handheld mode.— Shane Satterfield (@Dinfire) February 23, 2017
Nintendo already announced a day one update that will hopefully address this issue, although this issue wasn't specifically mentioned for the update. However, with how many reports are coming in, and considering the limited number of Switch's out in the wild at the moment, it's something to keep in mind as launch day approaches.
We have reached out to Nintendo for comment, in order to see if this was a known issue and if it is, if it will be addressed in the day one patch. We'll update this story should we receive a comment.