When you write a query you can of course make some mistakes. For example you could start a query with the negation operator -/ or open a tag group inside another tag group. Tagsistant will refuse to process a malformed query and will inform you about the error. This happens by listing only one file as result, called error which contains a description of your error:

$ ls ~/myfiles/store/{/{/t1/@@
error
$ cat  ~/myfiles/store/{/{/t1/@@/error
Syntax error: nested tag group. Close all tag groups before opening another.

This works only in the store/ directory because it's the only one that's supposed to return a file set, but it's also the one where the biggest part of your errors will happen.