What is another word for a false belief or opinion on something that people hold true and repeat because they have heard it repeated so many times?

It isn't any of the words on this page and I believe it started with the letter 'e' (though please offer your word even if it doesn't).

Example sentence: Because he researched what he had heard from his friends, he was able to refute many of their _________________.

Factoid seems to be the closest one, but the word I am looking for only refers to a belief that is false.

Solution 1:

I think the word misconception fits in your sentence:

Because he researched what he had heard from his friends, he was able to refute many of their misconceptions.

Misconception does necessarily mean false. Definition:

A view or opinion that is incorrect because it is based on faulty thinking or understanding.

Example:

Public misconceptions about AIDS remain high.

A myth is similar. Definition:

A widely held but false belief or idea.

Example:

He wants to dispel the myth that sea kayaking is too risky or too strenuous.

More synonyms include:

error, misbelief, miscalculation, false impression, fallacy, false notion, old wives' tale, urban myth/legend

(Oxford Dictionaries Online: factoid, misconception, myth)

Solution 2:

While not a single word, an old wives' tale may address the OP's query.

From the Wikipedia page:

Old wives tale is an epithet used to indicate that a supposed truth is actually a superstition or something untrue, to be ridiculed. It can be said sometimes to be a type of urban legend, said to be passed down by older women to a younger generation. Such "tales" are considered superstition, folklore or unverified claims with exaggerated and/or inaccurate details. (emphasis added)

One can infer that the false believe becomes more credible through repetition.

Old wives' tales originate in the oral tradition of storytelling. They were generally propagated by illiterate women, telling stories to each other or to children.

Solution 3:

1. Availability cascade

2. Bandwagon effect

3. Illusion of Truth Effect