At that performance of "Fiddler in the Roof" I saw Zero Mostel did things like forgetting the names of his on-stage daughters. It was near the end of his run in the musical. It happened so many times, Mr. Mostel kept it in because he got laughs. Lucille Ball did more out of desperation.
I am not sitting in a ivory tower, and certainly not the only person who had that experience with Zero Mostel. It certainly did not ruin the musical, but annoying -- yes, occasionally.