Mistakes in plural: why it's never "one phenomena"

What kind of error is it?

It is a grammatical and usage error involving incorrect use of singular and plural forms.

Why do people make this mistake?

People often get confused because "phenomena" sounds singular due to its "-a" ending, which is less common for plural forms in English. Additionally, the word "phenomena" is used frequently in speech and writing, leading people to mistakenly use it as a singular noun.

What is correct?

The correct singular form of "phenomena" is "phenomenon". "Phenomena" is the plural form.