Why "stoped" isn't stopping by your dictionary

What kind of error is it?

Spelling error involving incorrect formation of the past tense.

Why do people make this mistake?

The error occurs because of confusion about the rules for doubling consonants when forming the past tense. In English, when a verb ends with a single consonant preceded by a single vowel and the stress is on the final syllable, the final consonant is doubled before adding "-ed". Since "stop" is a one-syllable verb ending with a consonant "p" preceded by a vowel "o", the "p" should be doubled.

What is correct?

The correct spelling is "stopped".