Tomato, tomatoes, but never "tomatoe"!

What kind of error is it?

Spelling error.

Why do people make this mistake?

People often add an "e" at the end of "tomato" because the plural form is "tomatoes". This leads to the mistaken belief that the singular form should be spelled "tomatoe".

What is correct?

The correct singular spelling is "tomato" without an "e" at the end. The "e" is only added when forming the plural "tomatoes".