Here is something interesting, if you add Info() to the trim on B, it works fine:
Code:
A=Colorbars.KillAudio.Trim(0,-10).ShowFrameNumber # 10 frames
B=A.Trim(0,-1).Info() # 1 Frame
StackHorizontal(A,B) # Comment one of these out
#Stackvertical(A,B)
Edit: Even replacing Info() with AddBorders(0,0,0,0) would fix it.