When tunneling diagonally downward (i.e., not straight down) in the Nether, the player occasionally becomes "stuck" - as if stuck inside an invisible block, even when the opening is much greater than necessary. Player will also start to suffocate in some circumstances.

Player will continue to be stuck until all surrounding blocks - on all sides AND above and below - are removed.

This may affect areas other than the Nether and blocks other than Netherrack, but tunneling through Netherrack is the only place I've encountered this.

Bug began sometime during the 1.9.4 snapshots.

I will attempt to make a video of this issue and upload it.