Common mistakes, point deductions, and error codes follow. If you got some sort of letter-number combination in your comments, and you're wondering what it means, you're in the right place. If unspecified, an error is probably worth half a point, although half point errors may be nullified through good programming practices or if more major errors existed which caused your score to drop substantially.
G0) COMMENTS! Seriously, we like them. I grade 70 students' hw, I'm not going to spend that long trying to figure out what your code does. Also, comments should be semantic, don't just put comments like $t2=$t1-$t0; that's great, but what are $t0, $t1, and $t2? A good technique is to use C-like code with proper variable names or simple descriptions with proper names so that we can understand what's going on. It's not overkill to put a comment on each line, as well as an extra comment above major blocks of code.
Half a point may have been deducted for really bad commenting practice, i.e. either no comments in the entire assignment (mainly problems 3 and 4) or leaving off comments on a question you got wrong (well, more like writing comments might have saved you half a point). On that note, major props to people who heavily commented their code and even included C code and/or function declarations to explain what their code did. Super mega props to people who wrote Scheme code for Problem 4. And occasionally, if you made a minor error but your comment clarified what you were trying to do, I overlooked the error, so always write comments!