I propose that the course of development in first and second language acquisition is shaped by two types of processing pressures--internal efficiency-related factors relevant to easing the burden on working memory and external input-related factors such as frequency of occurrence. In an attempt to document the role of internal factors, I consider a series of case studies involving contrasts that are rarely instantiated in the input, yet show early mastery. I conclude with some general remarks about the nature of development and the possibility that it unfolds more uniformly than often suggested.