Water Pump Question

[Pete / MA]

I am rebuilding a water pump and when I was cleaning the housing I noticed a small hairline crack in the casting where the impeller bushing fits. It's about 1/2 inch long and seems to fade out to nothing. The 4-bolt mount, yoke and front bearing housing appear to be fine. My question, is this a lethal crack? What would cause something like this?
Thanks...................Pete