You need to maintain an index of the second array and dim/redim it as needed.
Code:

dim $BadItemsArray[10]
$BIAindex=0
For Each $item in $array
  ;do something that produces an error code
  If @Error <> 0
    $BadItemsArray[$BIAindex]=$item    ;Add item to bad items array
    $BIAindex=$BIAindex+1    ;check UBound here to ReDim
  Else
    Add item to good items array
  EndIf
Next
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