This is most likely a printing or camera-edge artifact, not a film-development defect.

From the answers, two common causes can produce this:

1. Camera/frame edge shape — Some cameras, notably certain medium-format models like Hasselblad V-series, expose a frame with small irregular cutouts at the edges or corners. If the print includes the full negative area, those shapes can show up as little image-filled corner protrusions.

2. Darkroom printing mask/carrier — When making bordered prints, the masking frame or paper carrier can leave a small gap or misalignment at a corner, creating a rectangular or pointed extension that still contains image detail.

Because the protrusion contains actual picture information, it’s not likely to be a chemical developing issue. Development problems usually show as stains, density changes, streaks, or blotches rather than a clean geometric extension of the image.

So the artifact was probably introduced either by the camera’s film gate shape or during printing, depending on how the original photo was made.