Film photography - what causes the following effect/error on a single frame of a developed roll of film?

Question

I shot the following picture with a Canon AE-1 Program on a roll of Ilford HP5 400 (I pushed the roll to 800). I developed it using Ilfosol 3 and scanned it with Epson scanner V370. As you can see, there are – I’m not sure how to describe them – smudge like shadows corresponding to the perforations. The shadows are only on the third frame of the roll. I am left to wonder what caused these on only one frame, and what can I do in the future to prevent it from happening?

The effect is even easier to see on the negative:

35mm developing film

The effect is even easier to see on the negative:

Steven Kersting · Accepted Answer

It's called "light piping" and it is due to some minor/cumulative light leak/exposure. Films with a polyester base are more prone to it, as is B&W positive film.
The only thing you can do to minimize the potential of occurrence is to store the film with as little exposure to light as possible (before loading and after exposure).
https://filmphotographyproject.com/content/howto/2019/05/light-piping-prevent/

Stan · Answer

As this pattern appears on a thin (underexposed) frame of the roll, I think it is due to slight over agitation in processing. The marks are characteristic of 35mm film over agitation.
Carefully inspect the rest of the roll and you will see the same effect on other frames coinciding with the sprocket holes.
The sprocket holes increase the flow of developer more than the unperforated edges due to a venturi effect.

Film photography - what causes the following effect/error on a single frame of a developed roll of film?

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