I've done this long ago to one of the companies I had worked for before – as in pointing out major holes, not taking over anything – but I think it's not a good idea if you don't have any previous relationship at that company and don't know anybody there. It's a huge risk. It doesn't give people a good feeling to be criticised, or to notice others are sneaking around their livelihoods... Imagine someone sneaking around your car to check if any doors are open. Even if you just want to helpfully point something out, you have given people a bad vibe that is surely not going to help you to get hired.