Garrison Keilor and Ira Glass have a fake feud going on, whereby they slip little insults to one another into their respective radio shows or public appearances. In Keilor's case, these often take the shape of mentions of Glass in his Prairie Home Companion sketches, impressions of Glass done by his actors, or pastiches of Glass' broadcasting style.
The ersatz Keilor/Glass feud is itself a shout-out to Jack Benny and Fred Allen, two giants of the Golden Age of Radio, who first hit upon this idea as a running (and recognised) prank on the audience.
Keilor also mercilessly lampoons other NPR figures in sketches, most notably news correspondents, who show up as voice impressions in PHC news sketches. Sometimes these pseudo-cameos are identified by name; often they are either un-named or given a ficitonal name, making the shout-out an inside joke among NPR staff and habitual listeners.