Validity of research on test subjects in psychological experiments

You may expect that I am going to write that I was doing some psychological experiment and that my test subjects were bugging me, but no, it's just about my experience in becoming a less-than-optimal test subject. At least for the typical researches I've been participating in for about a year now. Let's explain the story:

At the university you can sign up to test subject recruitement database which is consulted regularly by Human-Interaction researchers for recruiting the test subjects for their experiments. I did so about a year ago and since then I have been participating in numerous small experiments. Practially always I did not have a clue what the actual research was about and as such I was a perfectly well suited participant. But this week there were two experiment which looked a lot alike and they also looked a bit like experiments I have been in before, thus my judgement must have been a bit altered from the natural state. This threatens the validity of the reserach.

Actually, I am wondering how often this happens. Because once you join one experiment, it's easy to join others. But the first step is the difficult one. So, if you would like to help the researchers, and you have never participated in any of their researches, visit the recruitment database and sign yourself up! In retern for the experiment you get a small refund (in the order of 10 euros per hour)