Block programming has been suggested as a way of engaging young learners with the foundations of programming and computational thinking in a syntax-free manner. Indeed, syntax errors--which form one of two broad categories of errors in programming, the other one being logic errors--are omitted while block programming. However, this does not mean that errors are omitted at large in such environments. In this exploratory case study of a learning environment for early programming (Kodetu), we explored errors in block programming of middle school students (N = 123), using log files drawn from a block-based online. Analyzing 1033 failed executions, we found that errors may be driven by either learners' knowledge and behavior, or by the learning environment design. The rate of error types was not associated with the learners' and contextual variables examined, with the exception of task complexity (as defined by SOLO taxonomy). Our findings highlight the importance of learning from errors and of learning environment design.