WHEN YOU'RE GAMBLING that your illegally parked car will not be towed away before you can sprint in and out of an airline office to collect a ticket, you might decide to collect it first and read it later.

That's what I did, and that's where I went wrong. No, I went wrong a lot earlier. I forgot to ask a crucial question while I was still in the shopping stage. Now that I'm older, wiser and poorer, I plan to reform and to spread the gospel. It is this:

If you're a cheap-fare flyer, resolve now never, ever to sign up for a seat before asking, "If I have to change anything, is there a penalty or a problem?"

In my case, there was a penalty, and yes, the airlines can get away with informing you by writing it on the ticket after you've already paid. Sure, they should tell you up front. But sometimes they don't. And if things do come to a crunch, not looking at what you've bought until later means you are, in a word, stuck.

The thing to remember is that flying conditions are different here at the bottom. With reductions come restrictions, and never mind that what you hear today may actually have changed yesterday.

This cannot possibly happen in a great, big, up-to-date, computerized business; but since it does, it's a good idea to know what might be the case. For instance:

What do you mean there's a penalty if I want to use this ticket on another airline? Precisely that. A lot of low-fare tickets now come with a penalty for "changes," and changes normally include switching to another airline, changing to another flight of the same company, or changing dates.

Okay, but that's it, right? No. You see, airlines don't all have agreements to accept each other's tickets. Most do, but there are exceptions, and low-fare and small airlines have fewer agreements than the large, standard-brand companies do. That means that sometimes you can't switch to another airline without starting over and buying a whole new ticket.

You mean you have to go and ask the first company for a refund? Exactly. And if you paid for the first ticket with anything but cash, it will probably take six or eight weeks to get your money back. (If you paid by credit card, and then turn in your unused ticket, the airline will notify the credit card company, which will then credit your account. If you have no outstanding balance due, you can request -- preferably in writing -- that the credit card company refund the full amount to you.)

If you have to put out twice from a checking account, or use up too much of a credit line, that could be a bother. Which is why it's not the worst idea to ask one or two hard questions before buying. A Laker Skytrain ticket, for example, is usable only on Laker.

Then there's the World Airways example. Its tickets are accepted at face value for use with a number of other lines. But if you have to turn one in for a refund, there is now a 25 percent service charge for doing so less than seven days before the canceled flight.

Also, a ticket for a charter flight is a ticket for one particular charter flight, no other. (On the return portion of a round trip, though, the charter operator is technically allowed to let you make a change if there is an emergency. However, there's no requirement that it do so.)

So all right, sometimes you start over. What else? Well, let's refer again to the scheduled air scene. You may not really want to know this, but there can be a few other complications. For one thing, if you do change dates, flights or airlines, and the ticket has to be rewritten, in some cases you may be cutting yourself off from your low fare.

For instance, if you want to switch from one Super Saver fare to another Super Saver fare (and pay the change penalty), you still may not be able to do it. If both have rules requiring payment 30 or more days ahead and you try to switch only 15 days ahead, you miss the boat. You may have qualified for the first ticket but the qualification doesn't move with you. For the second ticket, you have to meet the requirements all over again.

That leaves you with three alternatives: pay full fare, go later or don't go at all! Maybe three and a half. The rules vary with the fare and the destination, but sometimes you can change the return portion of a discount excursion ticket without a penalty. Some Super Saver tickets, though, offer you only standby status as a replacement.

Isn't there some simple way to save yourself? A lot of people try to get around penalties and problems by buying trip cancellation insurance. Some kinds cover you in more situations than other kinds, but none are so comprehensive that you're covered if, say, a big business deal comes up and causes you to cancel. Basically, it's limited to death and serious illness to you or your immediate family. Whether the insurance is worth what it costs probably depends most on how much you have to lose. Some penalties, after all, are small.

Is there some other little something designed to trip up a cheap-fare flyer?

You mustn't think such ignoble thoughts. It is surely accidental that some of those fine folks touting spectacular standby prices fail to mention at all that coming back may be considerably more expensive than going. Capitol International, for instance, sounds like the low-fare flyer's true-blue friend with its $169 standby fare from New York to Brussels. That's one way, of course. Unfortunately, unless you ask, you might not learn that you cannot buy a return standby fare ticket once you arrive in Brussels. Thus, if you didn't know enough to buy a round-trip standby ticket in this country when you started, the cost of coming back with Capitol would zoom, up to roughly $264.

You're right, it's an ignoble thought. But is there anything else? Well, that's a matter of perspective. When push comes to shove and there's a long delay, a cancellation or a flight interruption en route, low-fare payers these days seem to get fewer of the goodies commonly passed around, like food, lodging and free phone calls. But, after all, that's how you have to expect the cookie to be crumbled. Does Hershey hand over the same size chocolate bar regardless of what you pay?