When I wake my Tamagotchi On from its resting mode, by hitting a button, before it fully turns on it briefly shows a horizontal line on the screen. Should I be worried?
That sucks. I would definitely return it. If you've lost the receipt try fiddling with the batteries as they may be the issue. My guess though is that it's a manufacturing error like eggiweg says. Sorry friend!
If it only does it sometimes, it might be that the ribbon cable connecting the LCD to the board is loose. This might get worse over time leading to a completely blank display, but your actual tamagotchi data should be ok.