At the end of mineshafts, the support beam is not up against the wall, even though there is a space hollowed out for the support beam to be in. This results in the support beam blocking the way of sections that branch off from one section.
Note
In the picture of the bug, the support beam has a block broken out of the middle. I believe this is intentional, to make the mineshaft look decayed and worn down. Ignore it.
Intended. Support beams do not need to be at the end of tunnels; they support the tunnel for some distance around them. The tunnel generated with that length rather than adding extra length to fit a support beam.