How does oil end up in coolant and/or vice versa?

I drive a 2000 Freightliner Classic and yesterday it developed a leak in the upper right region of engine...It was a very slow drip but knew it needed to see our shop...It was brown in color, felt like oil and smelled like coolant...My non-mechanical brain thought coolant leak dragging oil off of the engine...When I took it too shop, within a minute of looking I hear "crap"...He said, as he stormed off, that 'something' broke and cause oil to go into the coolant or the coolant to go into the oil...I didn't want to wait for him to come back and his attitude sucked so I figured I'd just leave it and go home...What could break to cause one of those fluids to enter the other system?...Oil pressure dropped to 35 on the trip back and the jake didn't sound good at the bottom end...Thoughts?...

 

That explains the expletive the mechanic uttered...

 

1.2m miles on it, sorry not sure of engine, I'll find out...