Although ID3v1 may well be easy to implement for programmers, it sure is frustrating for those with their own creative ideas. Since the ID3v1 tag had a fixed size and no space marked "Reserved for future use", there isn't really room for that much improvement, if you want to maintain compatibility with existing software.
A person who discovered a way was Michael Mutschler, who made a quite clever improvement on ID3v1. Since all non-filled fields must be padded with zeroed bytes, it is a good assumption that all ID3v1 readers will stop reading the field when they encounter a zeroed byte. If the second last byte of a field is zeroed and the last one isn't, we have an extra byte to fill with information. As the comments field is too short to write anything useful, the ID3v1.1 standard declares that this field should be 28 characters, that the next byte always should be zero and that the last byte before the genre byte should contain which track on the CD this music comes from.