Appendix D of F+O, in the entry for string-length(), says:

If you apply fn:string-length(@a) == 0; In 1.0 returns true if @a does not
exist. In 2.0 returns false.

This is no longer the case (we fixed this problem). string-length() is now 0, so
this expression returns true.

However, there's another minor incompatiblity that needs to be documented here,
which can be copied from the entry immediately below (normalize-space):

If compatibility mode is off, numbers and booleans will give errors for first
arg. Also, multiple nodes will give error.