What happens in your department when a member oversteps their boundaries ? (employee, application)

[AverageGuy2006]

What happens in your department when someone in your department handles work or a problem that isn't their normal job function or area of expertise ?

For example:
At the current company I work for, the materials groups has designated employees responsible for specific vendors/material types.
Employee A: primary material A/vendor A responsibilities
Employee B: primary material B/vendor B responsibilities

The only time there is overlap is when employee B covers for employee A and vice versa for vacation or sick days.

And although the designation may change from company to company, the concept remains the same. Specific employees often have specific responsibilities based off of logistics, vendor, or material/product application responsibilities. When attempting to contact materials group employees, they are very diligent in redirecting to the appropriate employee previously defined by management that is responsible for said issue.

However, I have noticed that in engineering, often engineers will take ownership of a problem that they don't have much experience with. I don't know if it is a willingness to tackle a new problem, learn more about a product, or an ego thing.
But certain engineers will hi-jack a problem even though they are not the primary contact, and they are not covering for another engineer. At the same time, usually the engineer will comment, "OMG, soooooo busy." But why take on problems that are not typically your responsibility then ?

Oftentimes, when issues and responsibilities are crossed in which someone doesn't have the product/task knowledge, mistakes happen at a higher frequency rate. Which in turn creates more work for the primary person responsible.