I see mainly two types of light leaks in
Primarily in vintage equipment there is a progressive or catastrophic deterioration of the fabric which leads to cracks, tears, and pinholes.
In more modern shutters this sort of major failure can also occasionally happen when the camera is exposed to severe conditions, contaminants, or destructive levels of air born ozone.
Usually a modern focal plane shutter is mechanically damaged either by being cut by some foreign material or burned from the heat of the sun being momentarily focused on its dark surface by the lens. During mid day in summer months at full aperture a severe burn can occur in a second or two with large aperture lenses focused on the fabric.
The only solution for old very deteriorated curtains is replacement.
For curtains in good health but damaged, patching is the most economical and practical solution.
Some stiff but not cracked fabric can in some cases be treated to allow using the original curtains.
In a SLR the curtain most vulnerable to burns is the second or following curtain which is exposed when the mirror is up in the unwound state of the camera if the camera does not have an instant return mirror.
Early Graflex shutter with original curtains
I reconstruct shutters for cameras including;
Leica, Contarex, Exakta, Kodak Ektra, Ermanox, Canon, Nikon, Ashai, Contessa, Zeiss, etc. etc.
ALPA REFLEX  with Kevlar ribbons