Although this also goes for massive LAN parties, it's especially appliable for private LAN parties.
When you are playing with friends at private LAN parties, they are most of the time held in relatively small rooms with bad air ventilation (like garages or living rooms). Sitting with at least eight people in a small room breathing the same air with running computers for a whole night long gives the air a nasty smell. When you are in it yourself you don't really notice it (ever been in a bar?). When there are smokers there (this is in 99% of the cases true) it makes it a lot worse.
Computers circulate air trough the case with fans to cool the components inside. If your computer runs all night in an enviroment like that, it has circulated a lot of that air trough it. So, the next day when you come home and turn your computer on for the first time, you'll get a blast of nasty air out of the back of the case. Usually it takes at least 2 days of full running time before it disappears.
example 2: When i turned my computer on for the first time after that LAN party i had to throw my windows open because of the LAN party smell.