Here's an example query that failed: https://en.wikipedia.org/w/api.php?action=query&prop=extracts&exsentences=5&explaintext=&titles=User:Pchelolo/Long_Test
Output:
{
"servedby": "mw1282",
"error": {
"code": "internal_api_error_Exception",
"info": "[V9MPcApAAE0AAc-HGa4AAAAN] Exception Caught: TextExtracts\\ExtractFormatter::getFirstSentences() error compiling regular expression /^(.+?(?:[^\\p{Lu}]\\.(?:[ \\n]|$)|[\\!\\?](?:[ \\n]|$)|\u3002|\uff0e|\uff01|\uff1f|\uff61)+){1,5}/u"
}
}