If you don't have the beautiful ordinance activated, your flowers will die and weeds will grow. If you're not careful, a rafflesia may grow. Also, if you're careless, your villagers will move out. So far nothing bad have happened to me hehe
Here's one that's specific to winter: If you TT backwards for even one day, your bingo card will expire. I think TTing hours is fine as long as you don't go past the last time you were on but I'm not sure. I also don't remember if the snowmen you built melt right away or not. I do know that I TT'd backwards and got so mad after my hardwork on my bingo card was wasted.