Considerable research has focused on the problems that end users face when programming software, in order to help them overcome their difficulties, but there is little research into the problems that arise in physical computing when end users construct circuits and program them. In an empirical study, we observed end-user developers as they connected a temperature sensor to an Arduino microcontroller and visualized its readings using LEDs. We investigated how many problems participants encountered, the problem locations, and whether they were overcome. We show that most fatal faults were due to incorrect circuit construction, and that often problems were wrongly diagnosed as program bugs. Whereas there are development environments that help end users create and debug software, there is currently little analogous support for physical computing tasks. Our work is a first step towards building appropriate tools that support end-user developers in overcoming obstacles when constructing physical computing artifacts.