yes in principle - as in you can overcharge something
but most modern chargers it senses 'full' and turns itself off
one particularly anal engineer I knew ( 'you dont have to talk to my husband everyone finds him boring' ) used to set an cooking timer for his charger and once interrrupted a meal so he cd rise and turn it off. I mean that is pretty anal innit ?
No nothing happens - when lithium batteries are charged they are charged. End of. Olden days batteries had to be run down first but that was the olden days.
Overcharging leads to Overheating of a mobile battery which reduces the lifespan of mobile phone so it is better to unplug when it is sufficiently charged. If you cannot be on your phone while charging you can use many charge notifier apps one of them is Full Battery Alarm.
Here is the link to the same https://play.google.com/store/apps/details?id=com.ebizzinfotech.fullbatteryandunpluggedalarm&hl=en