Probably close to 20 years ago at this point, when visiting the official forums for a game I loved, my computer was infected by malware delivered by a malicious ad. This was not some seedy part of the internet, but a website hosted by a major game publisher whose product I enjoyed.

Going to guess it was Blizzard? I remember a huge fiasco about that. If you visited their forums you were at risk of getting infected for about half a day.