In Poland there is such thing as probation in law, so you should specify what country are you talking about.
In German law there is. In Australia unfair dismissal can only be claimed after 6 months of employment. Not really sure about the UK, but I am fairly sure it's not just cut throat firing like in the US, and you can claim unfair dismissal.
UK you can claim unfair dismissal after 2 years, unless there's Equalities Act stuff happening.