If you're lucky, ESO eventually logs you out when you try to fast travel to a location that is overloaded (i.e. usually Rawl'Kha, Cyrodiil, sometime Craglorn). If you're not lucky, you just hang in the loading screen forever. It would be very very easy and so much nicer if the game simply aborted the fast travel, put you back where you started, and displayed a friendly message saying the location is busy - try again later.
As an aside, perhaps some balance changes should be considered to motivate people to spread out to other cities more., Rawl is popular because it is so convenient. Other cities are offensively inconvenient: wayshrines three miles outside of town, facilities too spread out, lack of full facilities, or having to enter/exit too many subzones to get anywhere (I'm looking at you Elden Root - the most irritating city of all, I think).