I have always had this doubt.
Which of these expressions is properly used and when to use each of them?
Debe haber sido un accidente
Debe de haber sido un accidente (is this a mistake?)
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In your example both forms are correct.
According to the RAE about the uses of
Debo cumplir con mi misión I must accomplish my mission .
In this case (denoting an obligation) is an error to introduce the preposition "de".
Debo de cumplir con mi misión (WRONG!!)
No hay luz en su casa. Deben de haber salido There is no light at their place. They must have gone out
Debe de haber sido un accidente.
When expressing a supposition, you can use both "deber de + infinitive" or just "deber + infinitive".
Marianita, su hija, debe tener unos veinte años Marianita, their daughter, must be twenty years old (expresses a guess, not an obligation)
Since you example denotes a supposition, both forms provided are OK.