Reported on #wikimedia-tech:
https://en.wikipedia.org/w/index.php?title=Special:Search&search=insource%3A%2Fmw%5C%3APageProp%5C%2FCategory%2F&searchToken=q7qlwhu465dkkxm5lps3z47w has ~15 entries at the time of this bug report. About 10+ are from today.
They correspond to the deployment today where we updated Parsoid HTML version to 1.6.0 and RESTBase also switched storage to Cassandra 3.