Involving end-users in the development of a product before it is deployed has great potential to increase the fit between a product and individual users' needs. While end-users can be directly involved in modifying low-fidelity prototypes, they are left out when it comes to high-fidelity interactive prototypes -in part because these cannot be modified directly or require time-consuming edit-compile-run cycles. High-fidelity prototypes, however, are more engaging for users. We created a reprogrammable high-fidelity prototype and explored its use in shortterm prototyping workshops with end-user developers, i.e. end-users with programming experience, in the domain of collaborative writing. We report observations and pitfalls, and distill four lessons learned into guidelines on how to use reprogrammable high-fidelity prototypes with end-users in contexts with limited resources. Our experiences demonstrate, among other things, that reprogrammable high-fidelity prototypes are difficult to work with -even for experienced programmers -and emphasize the need for careful attention to guiding participants, time for familiarization, and catering to multiple levels of programming experience.