This paper is a work-in-progress report on error annotation in the Lithuanian Learner Corpus (LLC), which has been developed using the TEITOK environment. The LLC is the first electronic corpus of learner Lithuanian that represents learners of very diverse native language backgrounds and different proficiency levels. In this paper, we have a double aim: firstly, we present the structure of the corpus in its current state; and secondly, we describe the main principles, procedures, and challenges of error annotation in the LLC. The main types of errors that are tagged in this corpus and analysed in this paper are orthographic, lexical, and syntactic.