I remember we played a prank on one of my colleagues back in the early 2000s who liked to waste time browsing ebay rather than working. So we changed the host file on his PC so that the ebay website would resolved to some pr0n website.
Then we all waited for him to fire up his browser and surf to ebay. You have never seen someone shut down their browser so quickly. None of us had considered the fact that his browsing history might get him fired, but he managed to get himself sacked anyway about a year later when he was caught nicking parts from the stock room.