This is one of my oak trees:

ftp://diy-efi.org/uploads/sgr/137_3781.jpg

The trunk that goes up and off the right of the picture has a large
crack in it, right where the first smaller branch is growing out:

ftp://diy-efi.org/uploads/sgr/137_3782.jpg
ftp://diy-efi.org/uploads/sgr/137_3783.jpg
ftp://diy-efi.org/uploads/sgr/137_3784.jpg

These are closeups taken from the other side.

I'm guessing the answer is yes, but does this trunk need to be cut off
before it breaks? This is a tree that my kids play around.

thanks,
--steve