An "invalid formatting" error is always because of the use of square brackets.
So as not to require much overhead, the software currently is doing a rudimentary check and doesn't know what the actual error is.
In essence, the software is counting brackets. Since this post has [two][sets] of open and close brackets, it passes the syntax check. An odd number of pairs would fail. Errors occur when posters use brackets instead of parentheses for parenthetical comments; using an open format tag and forgetting to close (for example, start bold without an end bold); some times it is just an extra bracket, (for example using two open brackets instead of one). The two biggest causes I see are improper editing of quotes (removing one tag and not the other) and using brackets instead of parentheses.
This simple check was instituted because these format errors were hanging the servers as the display engine tried to resolve the errors when it formatted a page.