Always quote all yaml strings. If you have a yaml file that has something that isn't a simple value (number, boolean) such as for example a date, time, ip-address, mac address, country code, phone number, server name, configuration name, etc. etc. then you are asking for trouble. Just DON'T DO THAT. It's pretty simple.

"Yeah but it's so convenient"

"Yeah but the benefit of yaml is that you don't need quotes everywhere so that it's more human readable"

DON'T