Programming skills are an increasingly desirable asset for higher education students across disciplines; however, learning to program continues to be difficult to master for many students. The heterogeneity of student cohorts in most computer science courses makes it challenging to isolate the concepts within programming that are particularly hard for all students to understand (i.e., 'threshold concepts'). As a result, there is a lack of consensus over the threshold concepts that are relevant to those learning programming. This paper draws on a qualitative study using focus groups with undergraduates and professional software developers to identify potential threshold concepts in programming. Data from the focus groups were thematically coded and analyzed using a theoretical framework based on established criteria for threshold concepts. In particular, we focused on concepts that were both troublesome and transformative and included other characteristics such as participants' partiality of understanding. Six potential threshold concepts were identified in the majority of the focus groups, including abstract classes, data structures and designing objects. Further analysis of the data identified additional concepts that may hinder rather than help the learning of these threshold concepts, which we have called 'accidental complexities'
