Context

I was running a series of tests last week, with 6 groups of 4-5 people each. Tests consisted in showing images to participants and asking a qualitative open ended question that had to be answered in a span of 20 seconds. Then a non-timed span for 2 questions using a Likert scale, and a non-timed span for a series of concepts using Semantic Diferential. All images for all groups were the same, only that in a completely random order. Tests were filmed, and there were 30 images per session. Average full session took 45-50 minutes.

Problem

I was running the tests and the Psychology Doctor who designed the tests was observing from another room (Gesell dome). After the tests were ran, she approached me and told me that everything was fine, but I shouldn't let users know the experiment was about to end (which I did with 2 or 3 groups around image 25).

I obviously respect her knowledge, but at the same time, I have observed that on those groups I told they were close to finish, they were less anxious. Today, while analyzing answers, I have noticed those groups gave better answers on the last images that those groups unaware of finishing time.

Question

Is there any protocol or study that expressly forbid to let participants know they're close to the end of an experiment? If so, are there any exceptions?