Grammar anomalies are frequent in texts produced by second language learners. When describing these anomalies, two main issues arise: What is anomalous use of grammar? And how are grammar anomalies distinct from orthographic and lexical anomalies? We review earlier error-definitions and suggest defining grammar anomalies according to an explicit norm instead of L1 usage. We propose a broader definition of grammar than in previous studies, based on Boye & Harder (2012). The distinction between grammar, lexicon and orthography is illustrated with data from 28 adult L1 English learners of L2 Danish. In the corpus, 55.9 % of the anomalies were related to grammar. Finally, we discuss how definitions and procedures can be used in future studies of naturally occurring grammar anomalies.