The recent JPEG2000 image coding standard includes a lossless coding mode based on reversible integer to integer filter banks, which are constructed by inserting rounding operations into the filter bank lifting factorisation. The baseline (Part 1) of the JPEG2000 standard supports a single reversible filter bank, the finite length input to which is symmetrically extended to avoid difficulties at the boundaries. While designing support for arbitrary filter banks for Part 2 of the standard, we discovered that reversibility is not always possible for even length integer to integer filter banks combined with symmetric pre-extension.