Same here. Just happened to me. I was busy enjoying the game for hours yesterday and forgot to redeem my tickets for the berries. So I logged into ESO after a long day at work to do my daily ESO "chores." The first thing I did was summon the cake. Without realizing I had 12 tickets I forgot to redeem.
Yes, many of us have lost two tickets and probably more than once, I have, I think. Anyway, since this is how it is, try to remember to check your ticket balance before you eat cake for the first time each day.
I have to wonder... why 12 tickets as the cap? What went into that decision? I can understand the rationale behind putting a cap on event tickets, but why 12 and not, say, 15?
My guess is...
(1) they want you to do the events. If you could just amass tickets you could completely skip events you don't like and buy vast quantities when there's an event you like. This way you are forced to be present throughout the YEAR more or less. It's a similar idea to Daily login rewards -- you have to show up or you'll miss out. More than that, if miss a few days you could completely miss the one or two better rewards they carefully place later in the month. ZOS wants you to show up regularly.
(2) 12 = 10+2.
10 is the price of items during the event, so they really don't even want you to be able to store anything.
The +2 is your overflow allowance. If you had, say, 9 tickets and you need 10 for an event item, and you redeem your Cake and get +2 tickets, you'd lose 1 ticket without any overflow allowance.
If you had 10 tickets and you "oopsie" used your Cake, you're capped at 12 and you better go to the Impresario.
I suspect what they really want you to do is immediately spend your tickets when you are at 10 or 11. When you need 50-90 tickets to get a fancy new mount skin and you can only hold 12 tickets... you are basically forced to show up every day. Or you can pay for the convenience of saving time and buy event tickets at 50 each.