Grammar or Spelling – Mixing Up ‘Quiet’ and ‘Quite’ Errors

Your question was," Is it a spelling mistake?" The answer is definitely no. It is a grammatical error, a capitalization error, but not a spelling error. When you capitalize a word, you don't change its spelling.

The phrase you're looking for is 'atomic typo'.

Urban Dictionary defines an atomic typo as:

A typo (one-letter keyboarding mistake, typewriter mistake) that the spell-check function cannot pick up because the word is spelled correctly -- although not for the word you wanted to key in. They are called "atomic typos" because the change of just one letter completely changes the word, although it remains spelled correctly in terms of the spell check function

Examples:

Chris, instead of Christ; war, instead of was; bite, instead of byte; massage, instead of message

There's an interesting blog post on the subject of atomic typos, at CentrEditing, which says:

The term ‘atomic typo’ was coined in 2002 and since then it has appeared in various blogs and newspapers but doesn’t seem to be in common usage (it returns only just over 3000 results on Google!).

There's even a blogspot dedicated to the atomic typo.