First let me say don't worry about it, but don't make a habit of it. The small amount that you have eaten will have a negligible effect. We all eat some potentially damaging substances with (almost) every meal, you just need to make sure that the unhealthy stuff is minimal in quantity.
When butter has gone "out of date" it may still be perfectly OK, however it is more likely to be going "rancid" in which case it has a sour taste, and contains some potentially harmful substances that are virtually absent in "fresh" butter.
I quote the following:
Rancid fat is the other bad fat -- fat that has been mutated by oxygen, heat, moisture, and light. This fat is full of free radicals and can contribute to all the health and aging problems associated with them. At first you might think that it would be easy to avoid this one. Just reading the word "rancid" tends to make our noses wrinkle in disgust; however, we have been conditioned to accept rancid fats. The reason that dollop of butter is salty is to help preserve it and to cover up the rancid taste. Yes, butter is good for us, but rancid butter is not. That flax oil in the refrigerator is liquid gold to some, but if it is rancid, it is worse than useless. We have to retrain our noses and taste buds to discern fresh good fats from rancid bad ones. This is one reason for using unsalted rather than salted butter.
from the foodreference.com site at:
Best wishes - Majikthise.
Nothing will happen to you, we had out of date butter in our house for a few months without realizing and everyone was ok.
No worries, I'm sure the worst you'll experience is a bit of a sick belly, and probably not even that.
The worst thing that may happen is an upset tummy but that's about it, I wudnt worry
You could potentially Die! -Better Bother to eat Better Butter! haha
Wuts date butter? butter made from dates?
Probably nothing but I wudnt do it
Butter that has expired
oh like rancid butter?
I Accedently Did !