Large crack in inner wing !

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Has anyone else had a large crack appear on the inner wing Unknw Sporty 1.6 1996 not used off road.
Couldn't believe it when i saw it, at the rear of the o/s inner wing from the top edge running right down to a pre drilled hole about 8" long.
I had to take the wing off and weld it from the outside, thankfully only 1 bolt sheared at bottom of wing. Is this a common problem or just an unusual fracture????
Anybody else suffered from this???

Gazza