One of the most common errors in Task 1 complaint letters is when the candidate uses overly aggressive language which sounds overly rude or excessively argumentative.  For example:

BAD EXAMPLE: Give me an answer immediately.

BETTER REPHRASED AS: I trust that I will be hearing from you in the very near future.

Being less direct and using less aggressive language and structures is very important. Avoid imperative structures like ‘GIVE ME’ and ‘TELL ME’.