If you’ve ever looked out the window of seat 43A at the tail end of the engine just after pushback, you may have noticed white smoke coming out of the jet and raised an eyebrow. The first time this happened to me, I had my finger hovering over the flight attendant call button until I realized nobody else was concerned; perhaps because they were all smarter than I; perhaps because they weren’t paying attention.
Schlepping around on The Straight Dope a few months back, I found out why this happens. When starting a jet engine, the mixture of gasoline to air is particularly high (think about the choke in your father’s 1978 Plymouth); this is necessary to get the engine to consistently start firing. As a result, some of the jet fuel in the mixture doesn’t get burned and subsequently gets expunged out into the exhaust.
You also might notice sometimes that only half of the lights on the plane function just after the engine(s) start. On some jets, after power is switched from the Auxillary Power Unit (APU) to the jets it takes a few seconds for the engine to power up and the alternators to start generating power. So sometimes when a single engine is running only that side will receive power.
More reason to have that bourbon before you get on the plane and to fall straight asleep; over analyzing the intricacies of aircraft operation and freaking out at every small hiccup will surely lead to an ulcer.