I worked on a fun project this week, restoring and hosting a friend’s html site that was a casualty of expired hosting. There were no recent backups. Thanks to a backup from 2013 and the Wayback Machine I was able to 100% restore the content. However, occasionally there were ads that would display. Adware? Malware. It was cross-browser. I ran all the tests. I looked at the code again. What could it be?
TUMBLR! Some of the background images are iframed from Tumblr. As part of their relatively recent move to showing ads, you need to to opt-out of on-blog advertising in your Tumblr settings. One I turned off ads, the ads went away.