Dragnet 67 to I Love Lucy

Dragnet spun off Dragnet 67

The main characters from Adam-12 appear as witnesses in an episode of Dragnet

Emergency! had a crossover with Adam-12 to find a boy who ran away from the hospital.

Both feature the fictional Trans Global Airlines

Diagnosis Murder's Dr Mark Sloan was defended by Matlock when Sloan was accused of murder

Mannix returned to TV on an episode of Diagnosis Murder to solve an old unsolved case with the help of Dr Mark Sloan.

Here's Lucy's Lucy Carter and Mannix's Mannix are held hostage in an episode of Here's Lucy.

Make Room for Grandaddy's Williams had Here's Lucy's Lucy Carter as a houseguest

The Danny Thomas Show came back as Make Room For Granddaddy

The Danny Thomas Show's Williams family shares a home (briefly) with the Ricardo family on The Lucy-Desi Comedy Hour in 1958, the next year the Ricardo family returned the favor and visited the Williams' on The Danny Thomas Show.

The Lucy-Desi Comedy Hour was a reworking of I Love Lucy