Adding a word accidentally to another, changing its meaning

A colleague accidentally keeps adding the word case after referring to a project called basket, thus referring to it as "basket case". Embarassing. But what is this linguistic phenomenon called, i.e. accidentally adding a word which changes the meaning so that the connotation becomes negative?

Malapropism

Though usually used to describe saying an incorrect word that sounds like the word you want, it applies to any sort of 'accidental misues of a word'.

I'd think your colleague is either committing a "Freudian slip" by sub-consciously equating the "basket" project with "basket case", which means someone who's mentally ill, or it's a simple "slip of the tongue".

Freudian slip: A Freudian slip is a verbal or memory mistake that is believed to be linked to the unconscious mind. Common examples include an individual calling his or her spouse by an ex's name, saying the wrong word or even misinterpreting a written or spoken word. (from Psychology.About.com)

Slip of the tongue: n 1. an unintentional utterance; a mistake in speaking (from FreeDictionary.com)