According to the BBC, Mars says it became “very clear, very quickly” that they had made a mistake.
Unfortunately, it’s still not clear if any products with animal rennet are already on sale, and Mars are being very vague about the whole thing, asking people to call their consumer advice line for information. As they haven’t actively denied products with animal rennet have been produced already, I can only assume that there are at least some batches of products out there that are not suitable for vegetarians.
Suffice to say, I’ll be avoiding all Mars products until they give clear advice on which products are suitable for vegetarians and which aren’t. Not because I’m a vegetarian myself, but simply because I like to know what I’m eating.