This arguably needs a Spoiler Alert. It's the last 25 seconds of "The Incredible Jewel Robbery," a half-hour episode of an anthology show called General Electric Theater. It was broadcast on March 8, 1959 on CBS and it represents the last real time the main three Marx Brothers appeared together on-screen. Incredibly, it was also the only time they all appeared together on any screen since A Night in Casablanca in 1946. In the interim years, there were a couple of projects in which all appeared (Love Happy, The Story of Mankind) but not in the same scenes.
The 1959 show was a funny caper film done almost completely in pantomime and starring Harpo and Chico as hapless crooks who disguise themselves as cops. Groucho, entering in the clip below for a surprise cameo, had the only line. (The man you'll see accusing them is, by the way, Benny Rubin who had a habit of appearing with great comedians and also being one himself.)
Shortly after this, the three brothers began filming on a TV pilot called Deputy Seraph in which they played angels. Halfway through shooting, production was halted because Chico was unable to perform. He had arteriosclerosis as was sadly evident in the few TV appearances he made before passing away on October 11, 1961. By that time, everyone had long since given up on Deputy Seraph. So since it never aired, these are the final moments of the Brothers Marx on-screen…