Losing a down

[ump33]

If you look at Case Book 3.5.11A ... 3rd & 8 from A's 45 when A1 throws and incomplete pass. The ball is erroneously spotted at the 50 for 4th down. K snaps & punts the ball into R's end zone. Per the Case Book Ruling, it is too late to make any corrections because the error had to be corrected before the ball was snapped.

If you adapt this ruling to the OP, the "4th down" counted once the ball was snapped and "it is too late to make any corrections because the error had to be corrected before the ball was snapped."

[ppaltice]

The case book situation you are looking for is 5.1.1.A. You can correct a down error up to either a change in possession or a new series is awarded. Dick Schindler defined 'new series awarded' as when the referee signals first down.

So in the OP case, if the referee signaled first down for B, then you can no longer make a down correction.