x='a]'
#
# \] is not a valid escape for ] in set glob expression.
# Glob sets have no escaping at all:
# ] can be in a set if it is the first char: []abc],
# dash can be in a set if it is first or last: [abc-],
# [ and \ need no protections at all: [a[b\c] is a valid set of 5 chars.
#
# bash-4.3.43 misinterprets [a\]] as "set of 'a' or ']'".
# Correct interpretation is "set of 'a' or '\', followed by ']'".
#
echo Nothing:${x#[a\]]}
echo Nothing:"${x#[a\]]}"
echo Nothing:${x%[a\]]}
echo Nothing:"${x%[a\]]}"
echo Ok:$?