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?
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)
You can find a taxonomy of speech errors (or "slips of the tongue") in Gary Dell's seminal paper, A Spreading-Activation Theory of Retrieval in Sentence Production on page 285 (page 3 of the linked PDF). The terms used are unsurprising and in many cases can be understood without explanation.
In this case, the speech error in question is a word addition. Word additions, like most speech errors, do not occur completely at random. Rather, a number of biases can be seen and patterns in errors can be identified. Since you've provided no context, the only relevant bias that I can identify is collocational: since basket case is a collocation, if an addition is going to occur after basket, then case has a decent chance of popping up.
Your question asks in particular how to describe a word addition that "accidentally ... changes the meaning so that the connotation becomes negative". That I don't have a word for, but if pressed, I might call it unfortunate.
Confusables: words that mean one thing when written as a single word and something a bit different when written as two words.
Typically in computer programming this means adding a string of characters to another string. I.E. Adding a word to an already existing word. For example: "basket " + "case" = "basket case". Though concatenation does not explicitly mean to "change meaning", that is most often what it implies.