Orphan trailing backslash in pattern
XCU 2.13.1 Patterns Matching a Single Character:
If a pattern ends with an unescaped <backslash>, it is unspecified whether the pattern does not match anything or the pattern is treated as invalid.
This means bs='\\\'; case word in $bs) echo match; esac should not match for any word. In existing shells, however, this case matches for one of \, \\ and \\\ depending on the shell.
Seems the future version of POSIX is going to say that the entire behavior is unspecified in such a case.
If a pattern ends with an unescaped <backslash>, the behavior is unspecified.