I was logged in as Irised and had adopted a pet (the parcel colour was blue), then I clicked the Advent button again and I was logged in as Leopardfire (and the parcel colour had changed to red). Nothing else happened, I was logged into my account on one page and then click I was logged into a strange account on the next page. I've never seen the username before, I've certainly never used it. The account is empty (whaa-aaat, I was curious!).
Sooo... I logged back in and hit refresh, it came back with 'error displaying pet' ... I clicked the advent button and I was logged in as Leopardfire again o.O ... refreshed the page again and I was logged in as Irised, so that must have just been cache (phew). Aaaand the present was blue with snowflakes again, and I successfully adopted the pet.
So that's my little story o-o