What word describes writing a word someone else said while trying to write something else?

I was trying to write "diagram" and someone said "poop" and I ended up trying to write "diapoop." I quickly caught myself, but I've always wanted to know what word is used for this phenomenon, if there is a word for it at all.

I believe what you are thinking of is commonly referred to as a Freudian Slip, also known as Parapraxis. Now, Freudian Slips are normally considered to encompass repressed or subconscious thoughts. Freudian Slips may also be due to the existence of some locally appropriate response pattern. In your example, someone said poop, and you started to write what you heard. Your response of including it in your writing could be due to a few things.

1) You could have been easily distracted at the time.

2) Hearing the word “poop” aloud may have been funny to you and in mentally processing the word, you physically wrote it.

3) You may have also been harboring a repressed detachment or disliking regarding the diagram and thus, appropriately wrote what you thought of it.

Freudian Slip - IntroPsych.com

Freud believed errors of all types were revealing. Like defense mechanisms, errors come in many varieties. Freudian slips are errors of language such as word substitutions and mispronunciations. Sometimes they involve sex, but that is not part of the definition. A Freudian slip is defined as any language error that is unintentionally revealing. Source Link: http://www.intropsych.com/ch13_therapies/freudian_slips_and_other_errors.html

Freudian Slip - Wikipedia.com

A Freudian slip, also called parapraxis, is an error in speech, memory, or physical action that is interpreted as occurring due to the interference of an unconscious ("dynamically repressed") subdued wish or internal train of thought.

Classical examples of parapraxes involve slips of the tongue and of the pen, but psychoanalytic theory also embraces misreadings, mishearings, temporary forgettings, and the mislaying and losing of objects.

In contrast to psychoanalytic theorists, cognitive psychologists say that linguistic slips can represent a sequencing conflict in grammar production. From this perspective, slips may be due to cognitive underspecification that can take a variety of forms – inattention, incomplete sense data or insufficient knowledge. Secondly, they may be due to the existence of some locally appropriate response pattern that is strongly primed by its prior usage, recent activation or emotional change or by the situation calling conditions.

This might be a bit more general than you want, but:

slip of the pen:

An inadvertent mistake made while writing, usually the substitution of one letter, word, or phrase for the correct one. Also called a... lapsus calami.

A slip of the pen is regarded as a type of heterophemy, the unintentional use of an incorrect or inappropriate word or phrase in speech or writing.

The expression slip of the pen dates from the mid-17th century.

(here)

Heterophemy is defined as "the (unintentional) use of some other word or phrase in place of the one that was meant" (here).

A heterophemism is presumably an instance of the general tendency.