The article reports and discusses a long-term qualitative study of forty 8-10-year-old students who regularly played a math game during math lessons for 9 weeks. The goal was to explore the relations between (i) some of the "pedagogical principles" that underlie the game and (ii) the "playing practice" in terms of what actually takes place when students play the math game during regular math lessons. The article discusses indications of matches and mismatches between pedagogical principles and playing practice as they appear in analyses of observations and video recordings. The result highlights the difficulty of predicting areas in which possible mismatches appear between the intentions of the pedagogues and designers of educational technology and the actual use of the technology by the students. This also applies to educational materials that have already been pilot tested and used on a smaller scale. We emphasize the need to observe actual use for extensive periods of time, i.e. to go beyond short-time user testing. (Contains 6 figures and 10 notes.)