I've actually had someone message me on Reddit (years ago) with several alt accounts that a reply I posted in a programming-related subreddit was wrong (it wasn't), and they were right, and they'd be mass downvoting my account. This was back when reporting actually did something I guess, because a couple of the alt accounts did get banned a few weeks later. Later another new account messaged me a censored screenshot of their bank account with a few million dollars in it and saying some crap along the lines of "I earned this money by selling a software startup, so I know more about C++ than you".

That's only one time I definitely know I was getting downvoted with alt accounts, but it really sucks. And now how do I know it hasn't happened several times since except with someone who doesn't have an inflated enough ego that they need to PM me proof of it.