Nothing that would have seriously impacted my ability to do a programming job, as it turned out; no enormous gaping holes. But there were plenty of little things I hadn't happened to encounter before, such as some of the finer points of computer graphics, a proper understanding of NP-completeness, functional programming and so on. I found it to be a mind-broadening experience and thoroughly enjoyable. (And the project I did probably resulted in me getting my current job, so even if none of the above had been true it might still have been worthwhile on that basis alone!)