An intermittent black shape on only some frames usually points to something physically obstructing the light path inside the camera, not a film issue. On an AE-1 Program, the most likely causes are a loose internal piece or a problem with moving parts such as the shutter assembly or mirror assembly. Intermittent faults are common with moving components because they may only swing or sag into the frame occasionally.

A loose flap or covering inside the camera can also cause this and may appear differently from shot to shot. Less seriously, an external obstruction like the camera strap could sometimes intrude into the frame.

If the obstruction is internal, it will likely become more frequent as the fault worsens. Inspect the camera carefully with the back open and no film loaded, looking into the film gate/light box for anything loose or out of place. If you’re unsure, continued use may make the issue easier to identify, but it also suggests the camera may eventually need service.