A computer-based information system is implicitly or explicitly subject to the labelling problem -- identifying the real-world object and establishing its cross-reference to its modelling entity stored in the database. Labelling errors in blood production process are very serious defects and pose great threats to blood safety, patient safety and the society. Yet, evidence shows that there are still blood donation centres using software with no measures at all or only insufficient ones against such errors. One of the possible causes why the software designer has failed to address this problem is the design barrier put up by computer?s obvious limitation on its ability to access user?s actions and circumstances. A better understanding of the user?s role in a computer-based information system helps the software designer break through this barrier and encourages him to seek better solutions.