This paper presents a study in which we examined spelling mistakes made by 34 learners of German in an online CALL exercise. We analyzed a total of 374 spelling errors that occurred in 341 words and subsequently classified them along four dimensions: (a) competence versus performance, (b) linguistic subsystem, (c) language influence, and (d) target deviation. We also evaluated the performance of a generic spell checker, one that is not specifically designed for second language learners, to determine the kinds and frequencies of errors it can successfully correct. Results indicate that 80% of the spelling errors in our study are systematic competence errors rather than accidental typographical mistakes. The study further reveals that MS Word 2003, the spell checker used in our study, fails to detect or provide a correction for 48% of the spelling mistakes made by our language learners. Our study offers explanations for the spell checker's failure to correct many of the misspellings and makes several computational and pedagogical suggestions to overcome some of the shortcomings of a generic spell checker in the CALL classroom.