In the series finale of The New Scooby-Doo Movies, Mystery Inc. finds Dick Van Dyke running a carnival, and, of course, he needs their help. Van Dyke had returned to CBS some time prior with The New Dick Van Dyke Show, which didn't have the staying power of the original (1960-6), nor was it in the same setting (since Mary Tyler Moore was doing her own Emmy winning series at this point). Van Dyke would return to Hanna-Barbera 20 years later to narrate the Christmas special, The Town That Santa Forgot, but this would be the only time he'd share the stage, if you will, with Scooby.
Edit, 4/23/18: The video has been deleted by Dailymotion, and no other copies are available at this time from legit sources.
Of course, the gang had some previous experience with evil at the circus after a fashion, but this was treated like it was a new experience for them. Ah, the lack of continuity was a bugger back then.