Negative transfer is a difficulty for Arabic-speaking students of English. This study juxtaposes eight categories of Arabic-to-English transfer errors with equivalent and near-equivalent errors in Arabic. It compares their error gravity as rated by teachers and students to discover whether there are any differences between teacher and student perceptions that might inform teaching practices. It is also the first step toward a cross-linguistic theory of comparative error gravity based on the functional equivalence approach to translation. It finds that teachers tend to perceive grammatical errors as graver than mechanical or lexical errors, with most differences in perception appearing to stem from cultural background. Among students, there is very little consensus, in line with previous studies. This study concludes that teachers and students might benefit from a better understanding of both their own tendencies in perceiving errors and those of the other party in the learning process. It also recommends the development of a full-fledged theory for moving between languages for the purpose of studying error gravity comparatively in languages other than Arabic.