SAN FRANCISCO -- Call it the carousel blues.

As the airport baggage claim carousel revolves, the gathering of faces hangs grim and anxious.Then a familiar suitcase burps out of the chute, and along with it -- somebody breathes a sign of relief.

But occasionally, your bag takes a little side trip without you.

Not often. "The good news is that 98 percent to 99 percent of baggage is delivered without incident," said Jim Ruppel, vice president of customer relations for Southwest Airlines. "The airlines -- Southwest and everybody else -- have a good record."

But now and then, bad things do happen to good luggage.

Airline representatives point to reasons your luggage may follow a different itinerary than your own.

One problem is time. Late check-in of luggage is a no-no. It cuts short the airline's handling time.

To think about the difference time could make, consider the netherworld of baggage handling.

If you've seen the movie "Toy Story II," you've got a clue. There's a scene in which the toys are riding conveyer belts -- some belts go up, some down, some zig this way, some that way.

"That is what it really looks like," said Andrew Plews, general manager of public relations for United Airlines. Big hubs like Denver International Airport have conveyer belts heading every which way -- carrying luggage, skis and bikes.

In many airports, bags travel from the belts down stainless steel chutes to carousels in the baggage handling area, much like the carousels we see on the customer side, reports Southwest's Ruppel. Bags are then sorted by baggage tag and put on the luggage truck going to the plane.

That little bag tag is tiny but mighty. It's your bag's ticket to ride.

Tip No. 1 from travel experts: Rip off those old bag tags. If your last trip was to Grandma's house, your still-tagged-to-Pittsburgh luggage may take another trip there, rather than the beaches of Bimini where you're now heading.

Tip No. 2: Always eagle-eye the agent as he tags your luggage, making sure that the tag really does say DIA for Denver International Airport, or whatever code for the airport you're heading to. Agents can make mistakes, and if you don't know the code, ask -- no matter how long the line behind you.

And here's another tip to avoid snafus: Make sure the bag tag reads your final destination -- even if you are switching airlines to complete your trip. Your bags should be ticketed all the way through.

"Where are you headed today?" -- the line from your friendly counter agent -- can innocently trigger a bag tag that lands your luggage in the wrong spot, says Ruppel.

"The customer may say "I'm going to Houston," because that's where they're getting off and changing planes, when in fact their final destination is Corpus Christi," said Ruppel.

Beyond that, luggage portage faces two issues: weather and a complex operation.

Ruppel says, "It's amazing to see the rampside of the operation. You could have a whole bunch of folks checking in for a whole bunch of different flights, flights departing to numerous destinations. Particularly at Southwest, you could have 12-15 different (final destinations) all on the same aircraft."

The plane lands, then the luggage is hauled into a handling area and sorted to 15 directions.

Compare the complexity this way: Think of the relatively simple household procedure of socks going into the wash. Washer, dryer, then sometimes they land back in the drawers as a pair -- and many times they don't.

It's the weather, stupid.

Most airlines would, if they could be so impolite, explain both delays and some cases of misplaced luggage this way.