What is the error called when numbers are confused with letters?

A homoglyph is a character identical or nearly identical in appearance to another, but which differs in the meaning it represents.

Wikipedia has a detailed article on homoglyphs as well as one on IDN homograph attacks where phishers take advantage of this property to mislead their victims.

As for your spell-corrector, you should be able to class such occurrences under something along the lines of homoglyph errors or homoglyph conflicts. However, it's a technical term which might possibly be a little too esoteric for your audience.

The error is called a homoglyph error.

Characters likely to lead to homoglyph errors are referred to as confusables in Unicode parlance.

The distinction is because glyphs depend upon the font as well as the character, what could be a pair of homoglyphs in one font, may not be in another (e.g. 0 and O look quite distinct to me right now, but may not in another font, while Τ and T look identical to me right now, but may be clearly distinct in another font).

Hence homoglyphs for the confusable shapes, and confusables for the characters likely to be rendered with them.