YouTube service stumbled on Thursday, leaving some visitors frustrated that they couldn't watch videos at the popular website. "We know some of you are having trouble watching YouTube videos," the Google-owned online video venue said in a message fired off on Twitter. "Our engineers are working on it; we'll let you know when things are fixed," the missive promised. Those who encountered problems at the website got an error page with an apology and playful assurance that "a team of highly trained monkeys has been dispatched to deal with this situation."