Tri-cocks, Tri-cocks, Tri-cocks!

Jim, what your friend had happen is living testimonial that anyone who operates a steam boiler needs to use the tri-cocks and use them frequently to verify that the gauge is not lying to them. You can bottom blow a glass all day and not free a plugged upper line. I've seen working engines with cobwebs on the gauge cocks and it scares me to death!
Guys and gals, if you're not using the gauge cocks, please use them. If you don't know what you're looking at when you open them get an old head to show you the magic quarter inch that tells you whether you're looking at steam, water or a mixture of both.
A water glass is a handy reference but it should not be relied upon!