1) Ideally, the experimenter is open to the possibility that the hypothesis is correct or incorrect.
2) In that case, there may be a psychological tendency to find "something wrong", such as systematic effects, with data which do not support the scientist's expectations, while data which do agree with those expectations may not be checked as carefully.
3) Sometimes, however, a scientist may have a strong belief that the hypothesis is true (or false), or feels internal or external pressure to get a specific result.
4) Another common mistake is to ignore or rule out data which do not support the hypothesis.
5) The lesson is that all data must be handled in the same way.