As a dev who does the client talking, sometimes it's really hard to avoid calling the client stupid. One time, while trying to confirm some stuff for a system, the following exchange happened:

M - Ok, so is this field mandatory?
C - Yes!
  * Shows off other stuff, comes back to that first field *
M - Alright, so the user will fill this field, which is mandatory
C - No! That field shouldn't be mandatory!
M - ? But you said it was?
C - No, I didn't!
  * Rewrite my notes, which the client saw me writing in the first place, stating the field is not mandatory. Proceed with the flow, come back to the field *
M - Alright, this field, is it mandatory, yes or no?
C - Yes!

There was also a different client that asked for a number of changes to a system, we did them, then, once he was supposed to test them give the final Ok, he came with "oh, we won't use that flow anymore, everything changed" - My boss chewed him hard and left his request at the lowest priority.