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Or during grinding. Or when the owner would accidentally tap the spine to the sink.
If this is due to drawing out the tang from the blade stock, the crack / fold could be due to hammering too much /...

Btw, that tang looks like a good amount of drawing out still needs to be done. This will make the crack worse if you don't fix it first.

I'm all for adapting the design when unexpected things happen, but a thumbnotch might not be viable if the flaw runs too deep. You need enough meat left for structural integrity, and don't forget...

Repair, no, not really unless you want to stick weld.

A closeup would be nice. This may be a flaw from drawing out the tang, where some bit of steel got folded inwards instead of drawn out....