Not sure about Austria, but in my home country, Poland, the employee is liable for statutory damages. He will be ordered by the court to pay the employer an amount equal to his gross salary for the notice period [1].

For example, if you earn 10,000 PLN gross (7,000 PLN net) and quit without observing a 3-month notice period, you will have to pay your employer 30,000 PLN, i.e. over four times the monthly net salary.

Of course, if the employer believes they incurred a loss due to the employee leaving without notice, they can sue for additional damages of any amount.

[1] Polish "Employment code" (Kodeks pracy), article 61, https://translate.googleusercontent.com/translate_c?depth=1&...

In the UK (as I understand it) you might become liable for the costs of replacing you until the end of your notice period, probably because of "breach of contract". But the business would have to be pretty shitty to do this and probably willing to lose all of that money again in legal fees. Mostly they would probably on refuse to give you a reference.