A mistake was made in the execution of the experiment. This may have been the fault of the technician who did not listen to his/her instructions or possibly the instructions were not sufficiently clear. Mistakes can occur in any study and so it is important that the person in charge of an experiment keeps a close eye on what is going on and makes sure that experimental data sheets are written neatly and kept up to date.
Likewise a biometrician needs to write down clearly (and keep a record of) the design of an experiment and any other instructions he/she wishes the researcher to follow. It is always a good idea for the biometrician to visit the experimental site himself/herself to see the layout of the experiment and to observe measurements being undertaken. He/she will better understand the nature of the experiment and the sources of experimental errors likely to arise.