Canon AE-1 - Manual (Page 62)

camera back and lift the cartridge out.
If you stop rewinding the moment the
frame counter has reached the "S" mark, the
film will not be completely rewound into the
cartridge and the film leader may still be
outside the cartridge.
mark, the manual aperture control "M" signal
above the aperture scale In the viewfinder
flashes on and off as a warning that the
aperture ring is not set at the "A" mark. (See
page 40.) ®
2 Did you set the film speed properly?
It is necessary to set the film speed
properly according to the film in use in order
to obtain the correct exposure.
3 ls the film properly loaded?
You can use the rewind knob as an
indicator that the perforations of the film are
properly engaged on the sprocket and the film
is actually advancing. Every time you advance
the film, the rewind knob should turn.