SHOULD AN earnest, upright traveler think evil thoughts?

Well, maybe a few. After all, it can't hurt, and it amay possibly help to venture off on a vacation with at last some handy hints on What to Do If Disaster Strikes.

Few masters of your fate intentionally play the "gotcha" game, but the way the system works - and doesn't work - some travelers inevitably end up holding the short end of the stick. You may also notice that it's not just the bad guys' plane that takes off without them. Nor is it rotten-to-the-core sorts to whom hotels say, "So sorry, we can't find your reservation." And perfectly virtuous people have had their baggage sent to Saskatchewan when they were headed for Salt Lake City.

For just such reasons, forewarned is forearmed. Here are some nuggets to squirrel away just in case:

If you miss the return trip on your charter excursion, you may not have to suffer the pangs of paying a full fare to get home. There is a little-known but very useful "Emergency Circumstances Doctrine" that allows the airline to bring a charter passenger back at no extra expense if it has room and the passenger has a good excuse. It's all decided case-to-case, but it means you don't have to give up without a try, even if it's a flat tire that threw you off schedule.

If you're on one of the new cheapie excursions with a scheduled airline, no excuses may be acceptable although some companies make exceptions for illnes or death - not just yours but your immediate family members' or even your traveling companion's. With proof, they may allow you to use your "unchangeable" low-fare ticket to come home either earlier or later without a penalty.

You needn't just smile and accept being bumped from an oversold scheduled flight within or serving the United States. You have rights, and if the airlines can't deliver you within a prescribed period, you stand to collect a full refund - in other words, a free ride. On Sept. 3, even stronger rules come into being - along with increased compensation. Even so, you don't have to take the refund that's offered on the spot. You can pocket the voucher but refuse to sign and think things over for up to 30 days without losing anything.

In the same situation, you don't have to stand around motionless either. If there's time and opportunity, you can ask - or ask the airline people to ask - if another passenger will change places with you. If so, the other passenger can receive in your stead any "denied boarding compensation" owed you.

Understand that even on tours, passengers who are bumped from scheduled (but not charter) flights may be eligible for denied-boarding compensation although they, like everyone else, must have met all check-in rules. That means don't fall in with phone instructions to "sit tight" at home or at a hotel if the flight is fouled up. Re-check until you're certain there's no oversale involved and that you don't stand to lose something by staying where you are.

Overbooking at hotels should be their problem, not yours. If they bounce you they ought to do well by you, so fight. But first be sure you've got a case by carrying with you your reservation confirmation. Without some evidence of conforming to their rules (including arriving by the time you said you would or they said you could), you've got trouble, so avoid it by being prepared.

Mishandled baggage is still a big, fat pain for both the transportation companies involved and the owners of the baggage, but it's more acute or the latter. It is also a somewhat gray area, with the rules too undefined to suit travelers done in by a delayed delivery rather than a confirmed loss. But squeaky wheels get most of the grease, so bring to bear all the power you can. Here it sometimes helps to holler to someone higher up in the responsible organization or to the proper regulatory body (either the consumer office of the Civil Aeronautics Board or the Interstate Commerce Commission).

In the United States, at least, you do not have to endure silently a train ride on which the air conditioning fails to function. When the thermometer goes over 80 and you suffer, you can claim at least a partial reduction on your ticket. Check the situation out with the conductor, then write to Amtrak.

Running out of money cramps most people's style. It's not your most pleasant experience, but contemplate the possibility and see what advice your banker has. Some will agree to respond to a telegram or telephone call with a coded, pre-arranged message. Alternatively, a family member can be alerted the same way and get off a domestic or international money order. Credit card and other companies may help cardholders. (Some card companies offer cash advances, others permit cashing of personal checks.) Ask questions before you go, especially if you're going far.

The fine print counts more than the big print in travel agreements - but not always, particularly when it comes to tours. Falling to fulfill obligations and hoping to get out from under by citing "no responsibility" clauses is not something tour operators can arbitrarily pull. More and more legal precedents are favoring the buyer such situations and even though it takes effort, it may be that a protest or a lawsuit is in order. Consult your local consumer organization or attorney general's office if you believe you've been done in.

If you're victimized abroad, your best bet may be to take your troubles to the local tourist office. They're likelier than the police to understand you if speak a different language, and in major cities more and more of them have complaint facilities with a specific office to send you to for help. This isn't a hard and fast rule, of course, but it's your best fallback position when there's no U.S. consulate around - and sometimes when there is.

These are parlous times and it's not always wise to accept one person's word regarding travel arrangements. Two areas, in particular, need long looks: (1) With air fares in a constant state of charge, it's practical to comparison-shop before you buy and maybe re-check before you fly, or at the last moment at which you can change plans without a penalty. (2) Internationally, airline baggage rules differ so don't just ask what applies on one portion of your trip, ask about each.

Knowledge isn't always power but it may get you out of a few sticky situations. As they say in the Caribbean: "You can even carry water in a basket if you know enough to put plantain leaves in the bottom."