Same thing for fellow vegetarians. Since you announced yourself vegetarian, then accidentally ate meat, what do you do?
1. Freak out.
2. Hate yourself for it.
3. Say you're sorry!
Just kidding! I know I've done all of the above, but the best thing you could do is...
RELAX.
It happens to all of us.
When I went vegetarian, I accidentally ate minced pork hidden inside my black bean noodles. I checked the ingredients and, although it didn't have any animal product written on the package, my tummy started acting up. I checked the website the ingredient "pork" was there. Horrified, I swore I would never buy that product again. And I learned to veganize that dish! The worst thing I could've done is, "Oh well, so there goes my vegetarianism, where's my double bacon cheeseburger?" and hop on the omnivore bandwagon.
Similar things happened as I went vegan. Heck, I once thought my order was vegan and offered it to all of my vegan friends! (It had eggs) I didn't mean it and I apologized soon after my realization. When I traveled to Tokyo, Japan, it probably happened more often since I couldn't speak the language. More on that later.
The sad fact is, we live in a non-vegan world, although there are lots of positive signs that more people are going vegetarian than ever. We are constantly bombarded with hamburger signs, BBQ outings, veg-unfriendly restaurants, and friends & family (except for vegan friends, of course) that meat and animal products is a way of life, thanks to years of advertising by the ultra wealthy meat & dairy industry.
But it doesn't have to be. At least for you, me, millions of other vegans around the world, and all animals.
Every mistake that I've made along with my wonderful journey of veganism was a great learning experience, and I know I won't make that mistake again. It made me review my stance on veganism and pray to every creature around the world to have a better life in the future.
Also say sorry to my beloved tummy. I will treat you better next time!