Originally posted by HarmlessYoda:

I have 3 separate pens for pigs, the only time a pig "gets out" is when they give birth to a baby piglet with their rear end up against the fence. The piglet then appears on the wrong side of the fence which eventually means I end up with tame pigs roaming around inside the rest of my base instead of just in the pen. Not really a big deal.

This^
Happens too me all the time. If you have an elevated breeding stand to a piglet drop pen be sure there is no way the piggy can catch on something and get outside where you want them. This vid addresses the "dropping outside" issue starting about 8:45.
https://www.youtube.com/watch?v=RDVUKf1TLxo