POSTAGs are a feature of the morphological dictionary that catalogues the words by their use in the language and tags them in relation to one another or to a common base form. Some rules are based on these tags instead of words, so they can apply automatically to all words (or word relations) of one group.
All datasets have errors and this is no exception. The most ubiquitous POSTAG errors have been found and fixed but there a still a few hundred there to be found. That is not much (considering the hundreds of thousands postags), but is a too daunting task to systematically find all these misbehaving items, so, any error reporting of this kind is welcome.
If you found one or you have a rule that should be working and it is not:
* Paste the phrase on text analyser (http://community.languagetool.org/analysis/analyzeText)
* Click ‘Show analysis’;
* Verify in your dictionary that that form does not exist (or that is in fact missing);
* Post here like this: word (token); base form (Lemma); wrong tag (abbreviated on Part-of-speech).