It is sometimes possible to attack from blockstun. A phantom hitbox will come out and hit the attacking opponent but you will remain in blockstun. It seems to happen when buffering a normal from blockstun but you are put back into blockstun on the last possible frame. So far I only have evidence of this happening in training mode using the dummy function to do a reversal throw from blockstun, however I have witnessed this happen in online matches.
The easiest way I have found to recreate this is to select Geiger as Player 1 and DeGrey as Player 2 in training mode. Set the dummy to Always Block and to Throw After Blockstun. With Geiger do jumping A, neutral A cancelled into B. DeGrey should move forward while still in blockstun and Geiger will get hit or block a phantom hitbox.
Here’s a youtube video showcasing the glitch and the steps to recreate it: