When capitalizing with the shift key sometimes everything after this is also capitalized, as though the shift key was held down continuously. This also applies at times to contiguous selections, such as photos or text. I spoke with Apple Care service, and they recommended changing the Bluetooth keyboard, and promptly sent a new one (under warranty). Problem presisted.
So once again sought advice and after the 'new user" routine (where the problem persisted), went back to administrator user (me), and changed the settings in system preferences/keyboard and mouse/keyboard, ot reduce the "delay until repeat" slider to half way, but left the key repeat rate at one less than "fast". This helped solve the problem and made good sense, given there seems to be a slight delay in keyiing and action on the sceen, but this is unpredictable and intermittent. It didn't matter in which application I was typing. It occasionally happened with the command key as well.
So if anyone has this problem try this simple fix first.
Don