As reported by ralfh in https://lang.moodle.org/mod/forum/discuss.php?d=7378

I've seen several strings with an ending \ like in '{$a}\.

I just tried an AMOS search of all strings in 3.7 for {$a}\ and found 94 (all non-standard plugin strings) ending in '{$a}\ (with a backslash instead of a single quote).