Accusative I involves the usage of the first person singular nominative subject pronoun 'I' as an accusative object pronoun. For example, %"He gave it to John and I" - or %"it is I".
This often comes from hypercorrecting the correction of nominative me to 'I' to accusative cases. Ugh. It's often used by native speakers who are confused by grammar.
This is usually considered an error, especially in formal registers.
In comparisonal uses, e.g. "John is as tall as I" or "John is as tall as me" are both commonly accepted. Some prescribe the former, based on elision of a verb, e.g. "John is as tall as I am". But prescriptionists, even more ugh.