If you've been charged for damage you didn't cause, the circumstances are really important:
The damage was caused whilst you weren't with the car:
Usually, if damage is caused to the car, even if you weren't there when it happened and it wasn't your fault, you'll be held responsible for the repair costs. Examples might be where the car was hit by another in a car park, or when a tyre deflates whilst it's been parked outside your hotel. In these circumstances, you will usually pay for the damage and reclaim this from us.
The damage was already on the car when I collected it:
Under the terms of almost all rental agreements, you are responsible for any damage caused to the rental vehicle that wasn't there when you collected it. The way that this is proven is by checking the 'check out' document (the paperwork given to you when you collected the car). If the damage isn't listed on here, or drawn on to the diagram of the car, you'll be responsible for the damage when you return the car.
I was hit by a 3rd party (another person)
If you were hit by a 3rd party, you should usually contact the Police to obtain a report that shows you were not at fault. You must always make sure you get the details of the 3rd party (providing it's safe to do so) and you must report this to the rental company as soon as possible. It's not uncommon for the rental company to charge you the full excess for the damage in these circumstances until they are able to recover their costs from the 3rd party's insurers. Usually we would not process a claim under these circumstances until you had proof from the rental company that they had not been able to do this.
There was no damage on the car, and nothing noted when I returned the car
When this happens, assuming you have a signed copy of the return document from the rental company that confirms there was no damage, you shouldn't have been charged. Under these circumstances, we would not be able to pay a claim and you would need to take this matter up with your credit card provider or local authorities.