Programming-by-Example (PBE) enables users to create programs without writing a line of code. However, there is little research on people's ability to accomplish complex tasks by providing examples, which is the key to successful PBE solutions. This paper presents an online user study, which reports observations on how well people decompose complex tasks, and disambiguate sub-Tasks. Our findings suggest that disambiguation and decomposition are difficult for inexperienced users. We identify seven types of mistakes made, and suggest new opportunities for actionable feedback based on unsuccessful examples, with design implications for future PBE systems.