A titration lab involves using a solution of known concentration to react with a solution of unknown concentration in order to determine its concentration. Most of the sources of error in a titration lab will be associated with human error. There could be an error with making up the chemical solutions or with measuring out different volumes. There could be an error with reading the buret volumes or with adding the two solutions together to the equivalence point. But these are all sources of human error. About the only source of non-human error that I can think of would be if any of the glassware was defective, especially if the buret markings were incorrect.