Acting for Two: Chespirito in the Cross Over; Carlos Villagrán in a flashback with Quico's father and baby Quico; María Antonieta De Las Nieves as both La Chilindrina and her "Bizcabuela"; Edgar Vivar as Sr. Barriga and Ñoño (sometimes even Talking to Himself); Florinda Meza as Doña Florinda and look-alike niece La Popis.
The Danza: "...Florinda Meza as Doña Florinda; Ramón Valdés as Don Ramón..."
In later episodes, Maria Antonieta de las Nieves (La Chilindrina) as Doña Nieves.
Chespirito liked to do this, because he felt the actors would be more connected to their characters this way. Same went for many of them when they played extras in El Chapulín Colorado, which used the same cast.
Missing Episode: some of the original early sketches. And depending on what country you're in, many episodes can be missing from syndication. Brazilian TV network SBT (heavily associated with the show, having aired it almost nonstop since The Eighties) annouced recently that it will be airing some "missing episodes" never seen before (or which haven't been shown in a very long time) in the country.
Despite the success of El Chavo, the series had its detractors, mainly for their slapstick scenes that some considered as "violent" and harmful to children.
Ecuador takes this trope Up to Eleven. Ironically, Ecuador being the first country where the program was successful outside of Mexico, is the same country where the strongest criticism appears. Some of the comments against the program come from Ecuador and studies on the incidence of violence on TV (where they include El Chavo as an example) are from Ecuador.
Reality Subtext: many of the cast departures were for internal personality clashes.
Recycled Script: Some scripts were done as much as four times. Sometimes, with the very same characters in the very same roles.
Talking to Himself : In the later seasons, (after the departure of Ramón and Quico) most of the cast actually played two characters. And they were needed to interact each other using split image. In the Cross Over with El Chapulín Colorado, even the titular character needed to do it.
Throw It In: Many bloopers made it to the final edit. Sometimes this was because they were funny or the cast used them to their advantage. In one episode, after a typical confrontation with Doña Florinda, Don Ramón angrily throws his hat, and it accidentally goes through Doña Florinda's door just as it closes. Don Ramón continues his snit fit but briefly pauses to knock on the door until one of the other actors lets him in to retrieve it. (See it here.) No one breaks character, which makes the scene funnier than if it had gone as planned. Other times, production problems that should have caused a stopdown didn't. There is one early show where a loud thunderstorm is obviously going on outside the studio, and yet no one mentions the frequent thunderclaps that can be heard over the dialogue.
In one of the versions of "Que Bonita Vecindad", Ramon Valdez clearly messes up a dance move, but it was left in the final version.
Throw it ins often got through because of schedule deadlines rather than for being funny since Chespirito was known to dislike improvisation.
Also, famous soccer player Pelé once phoned in Chespirito to ask him to make a movie of the show and even offered himself to appear in it, but Chespirito declined as he didn't support the idea of having movies of shows you can see for free on TV.