I received a report from a user:

"An ad by Eggland's Best periodically pops up and wipes out all the progress on the current puzzle."

Has anyone else seen this?

I disabled ads completely for now until I figure out what is going on

With some checking and testing we figured out this is different software (malware)
injecting ads into other pages / web sites:

the ad was a "pop-over" which took over the entire page, and this is a type that is not used on the site.

The only types used on the puzzle page are the vertical ads on the left and right hand side, and the almost square ones to the right of the puzzle.

So if you do see pop-over ads, or really any ad that covers the page, then you
should check your computer for viruses/malware.

(or switch to using a Mac / Linux )