On a KNITTING website, of all places.

A pop up allegedly from Adobe Flash, tried to download an update. I declined the request. As I watched, up came a screen telling me where to put the "download". Then a "warning" from "Apple" that this Mac has been infected and I should phone a number.

Firefox history shows the trail: this one shows against "Official Apple Support"

http://systemuseasurement.club/........www.knittingpatterncentral.com

Shall I tell Knitting Pattern Central that their site contains potentially malicious bugs, or not bother?

There was no download, in case you're wondering. Either I stopped it or it was a scare tactic.