Another error, this time from Burrow, set the Cowboys up again, with Kyle Feldt the try-scorer.
A handling error, this time by Anthony Stewart on the Reds' 30-metre line, gave the visitors a great attacking position.
Another handling error, this time from Robbie Paul, then gave Graeme Horne an intercept try from halfway and, for the first time, Hull had some daylight at 18-8.
Having won the first leg 28-13, Bristol had a certain margin for error this time but the ghosts of their grim play-off final defeats to London Welsh and Worcester in the past two seasons remained stubborn companions.
Chelsea had drawn with Reading in the corresponding fixture last season, on Boxing Day - a result that they felt keenly in the final reckoning - and are keenly aware that they have no margin for error this time out.
The momentum had taken a decisive shift and another handling error, this time by Kris Welham, gave the Wolves the position from which Ben Currie, switched from the second-row to the three-quarters due to injury to Ormsby, took Myler's pass to score his side's third try.
A second defensive error, this time a weak back header from Potter, allowed Bradshaw in to double Walsall's lead.
To make one mistake like that is bad enough, but last year Thoman made the same kind of error, this time with the sales force.
Since Leica Geo Office provided only uncertainty of baseline length as the observation error this time, we used this value as the criteria of observation error.
There can be absolutely no room for error this time around with that which we seek to obviate.
