The syntactic errors adults make in learning a foreign language are often those native children make as well. These “analogy” errors are just as important, as the adult progresses towards mastery of L2, as “interference” errors based on his L1. The former errors actually increase as learning progresses while the latter decrease steadily. The two contributions to a syntactic error can be measured by asking L2 and L1 speakers, respectively, whether a sample sentence is a plausible native utterance (literally translated for the latter). Judges' ratings of 40 common errors correlated .41 and .47, respectively, with the probability of actually committing them (R = .59). With students' L2 proficiency taken into account, along with the two kinds of ratings, syntactic errors were highly predictable (R