Ha! I just had the same thing happen to me--and I've been around long enough to always be logged on. I put someone else's greeting card in my shopping card, but it listed at my price as if it was my card--and a framed print of my high school that I intend to order this month was marked down. I have no idea what happened! :-)
I've just noticed the same - I recently did order a print of my own work, then today while logged in, I clicked on the print to check one or two details, and it showed a 58% discount, with 15 hours remaining on the sale and the regular price crossed out in red. Other print sizes and options also show a seemingly random discount. Then I clicked on other artists' work, which also showed discounts, but this time 20% for all sizes and print options.