Inspired by this question:

[substitute "6 hours" in this question with whatever one's tradition might be]

Suppose it is less than 6 hours since Reuven last ate meat. Out of habit, he mistakenly bites into a dairy candy bar. While it's still in his mouth - but before he swallows it - his kind friend Shimon reminds Reuven that he's fleishig.

Now Reuven is in a dilemma: Does he spit out the candy bar or swallow it?

I would expect an answer treat the type of food as an example, and treat all cases (e.g. what if it was milk, etc.)

So what should Reuven do?