What happened in the OP is a bad shot not a bad roll.
A bad roll occurs when an unexpected event occurs on an otherwise good shot. These things come in two levels, one that could have been predicted, and those that almost assuredly could not be predicted.
In the OP, the opponents OB was surely visible at that end of the table when the 3-rail bank was attempted. Not seeing it and not accomodating where it was is bad judgement not a bad roll.
Now, if the opponents OB was up table before the shot, and something unpredictable about the shot rolled it down table and the CB ended up behinid it, then it is a bad roll situation.
A bad roll occurs when an unexpected event occurs on an otherwise good shot. These things come in two levels, one that could have been predicted, and those that almost assuredly could not be predicted.
In the OP, the opponents OB was surely visible at that end of the table when the 3-rail bank was attempted. Not seeing it and not accomodating where it was is bad judgement not a bad roll.
Now, if the opponents OB was up table before the shot, and something unpredictable about the shot rolled it down table and the CB ended up behinid it, then it is a bad roll situation.