Whenever I drink and smoke.
One time I had just a beer and a couple tequila shots, started smoking a joint and got a phone call from a friend (I'd say it was about 1 in the morning) well I realized how fucked up I was trying to carry out a conversation. Then I had another beer and smoked more, lost all sense of where I was and what I was doing. Got the spins, puked in my bathroom and sat down with my eyes closed for a while before deciding I better go to bed.