Right now, every error message contains the position of what is wrong. However, that position may sometimes be confusing. Here are some examples:

• Write a filter with content foobar. The unrecognised variable message will mark as position the end of the string (would make sense to mark the beginning)
• Write a filter with content /*. The unclosed comment will be marked at character 0 no matter how many whitespace there is before it (actually, this was introduced by me with https://gerrit.wikimedia.org/r/#/c/425321/)

More generally, the reported position is the end of the 'wrong part' (as a side comment, Ace doesn't move the caret if the position is equal to the very last position in the rules string). Although T187686 may solve at least a part of this problem, in the meanwhile we should make reported positions more useful.