The regular expression to find Go test errors is currently (

1. The \s-+ also matches newlines (at least when using the standard syntax table).
2. The [^()\t\n]+ group matches a ton of things: spaces, exotic characters, NUL bytes, ...

Combined, these two mean that e.g. a GNU-style file:line:column: message line that follows a newline is matched incorrectly:

(with-syntax-table (standard-syntax-table)
  (let ((regex "^\\s-+\\([^()\t\n]+\\):\\([0-9]+\\):? .*$")
        (line "\nfile.go:1:2: word word"))
    (list
     (string-match regex line)
     (match-string 1 line))))

⇒ (0 "file.go:1")

Here, this incorrectly treats file.go:1 as filename and 2 as line number.
This causes issues because such lines are very common in practice, and the leading newline alone is enough to trigger go-test matching.

I recommend restricting the go-test pattern:

1. Replace the \s-+ by a more precise match, e.g. four spaces.
2. Replace the filename group by something that is likely to be an actual filename (e.g. no spaces or colons).