Buying insurance for your holiday can often be an after-thought, but the potential problems and expense that can be caused by getting it wrong could result in your holiday being one to forget rather than remember.

Here's six mistakes to avoid which might invalidate your policy.

Get Drunk

It's something of a myth that all travel policies are invalidated if you consume alcohol. Most allow for "social" drinking and even if that isn't stated in the policy, most insurers won't hold it against you if you have a quiet drink and are then injured in an unrelated accident.

That said, if it can be proved that alcohol contributed to the reason for a claim, the insurance company could either limit the pay out or reject it alltogether.

If you intend on driving or motorcycling on holiday, it's usually safest to avoid any form of alcohol on the same day as any hint that you were driving under the influence is likely be mean a quick end to any claim.

Use Illegal Drugs

Unlike alcohol, this is almost certain to invalidate your policy immediately. In some cases it may even mean a double invalidation if your policy bars both drug taking and illegal behavior. You may also need to check your policy to see if it excludes taking drugs that are illegal in the UK but legal in the country you are visiting. And remember that in some countries, particular drugs may be decriminalized rather than legal: that means you won't be arrested, but you could invalidate your policy.

Fail to declare an existing medical condition

As a general rule, travel insurance doesn't cover situations related to undisclosed pre-existing conditions. These conditions are normally only covered if fully disclosed, which may mean a higher premium or taking out specialist holiday insurance for pre-existing medical conditions.

Exactly how this is enforced depends on the policy and the insurer concerned. Usually if you know about a condition, it has to be declared. If you've suffered symptoms as the condition developed, but the condition hadn't been diagnosed, it usually comes down to a subjective assessment of whether you should reasonably have believed you could be suffering from the condition.

Take Part in Extreme Sports

This is another area where it really does vary from policy to policy. In the small print you'll often find a list of prohibited activities, which is well worth checking. Typical policy exclusions are water based sports, climbing or any activity that requires your feet to leave the ground at any time! One exception that often catches people out is using a motorbike or even a moped.

Extend Your Trip Without Notice

Single-trip policies are only valid for the designated dates, and not a day longer. If you want to extend your trip, an insurer will nearly always agree to do so (for the appropriate extra premium) as long as you contact them. However, if you make a claim for an incident that happened after your policy expired, you'll be out of lucking if you are expecting some sort of grace period.

Take A Day Trip Across The Border

If you have a truly global policy, you can go anywhere (with the possible exception of countries on the Foreign and Commonwealth Office's travel advisory list). But if you have a policy that restricts you to a geographical region, be careful not to breach it. For example, if you go to Southern Spain with a European only policy, you take that ferry to Morrocco at your own risk. If you do have a specific European policy, check carefully to see how it defines the restriction: for example, a "European Union only" policy won't cover some countries on the continent.

Being aware of the exclusions and limitations of your travel policy can help you avoid further stress should you need to claim or give you the chance to secure wider cover. So be sure to know what cover you do need and read the small print before you buy, and you reduce the chances of falling victim to small print exclusions.