A freak of nature??

Pulling apart a bit of splitting tongue and groove planking left over from making my roof box, I got a surprise.

Here’s the split.

and when I pulled it apart:

Well did you ever!  A fully formed little branch growing inside the plank.  Note the groove where it sat in the lower piece, the whole thing over a foot long.  Is this common?  I never came across it before. Do branches grow inside trees?  Feel free to educate me if you know more about these things.