If you have taken advantage of unicode parser feature through explicit or implicit use of the U&'syntax' (in a stored procedure, for example), the text of the stored procedure is saved in syscomments. If you roll back to 12.5.0.3 and the server needs to recompile from query text, you get syntax errors.
On another note, if you take advantage of the fact that in 12.5.4 unichar no longer requires UTF-8, you may have stored procedure plans that contain unichar-to-char conversions. In 12.5.4 this is handled correctly on a non-UTF-8 server. In 12.5.0.3 and earlier, however, the assumption was UTF-8; anything else causes the conversion to throw an error and abort the transaction.
