I can't come up with specific examples of peasants but in German cities with an imperial charter, there was a custom of "Stadtluft macht frei nach Jahr and Tag" (in English: city air makes one free after a year and a day). Implying that a peasant or serf who came to that city and could effectively evade their feudal lord for such a time would be granted their freedom and could in theory rise well above the station they were born in.