In clinical psychology, a slip (also sometimes called a lapse) is a problematic action during, or more usually after, a psychological or medical treatment. It happens when a patient, during or after a treatment to modify a maladaptive behaviour pattern or habit (such as a drug addiction), does an action characteristic of the problem being treated. It is different from a relapse in that a slip is something puntual, that happens once, and does not imply a regression to previous (maladaptive) behaviour patterns. Slips are somewhat expected after a treatment, depending on the problem the patient presents.
Warning the patient
It is very important to warn the patient of the possibility of slips during or after the treatment: not doing so could result in the patient thinking that because he/she has slipped, the treatment has failed and there is nothing to do now, thus completely reverting to previous maladaptive patterns (which could then be defined as a relapse). For example, when a bulimic patient has eaten a cookie outside the prescribed diet (slip), he/she might think that now everything is ruined because the treatment has failed, and proceed to eat everything that is in the fridge and later vomit it (relapse).
The psychologist or mental health professional should establish the difference between a slip and a relapse before the beginning of the treatment, so the patient is aware and prepared to face a slip with the knowledge that it is puntual and does not necessarily mean that the treatment has completely failed or that the patient lacks self-control: on the contrary, it should be perceived as a chance to put into practice the psychological techniques learned in therapy and fight the possibility of having a (much more serious) relapse. Extra therapy sessions that focus on the causes and ways to avoid that particular slip are then recommended in order to eliminate or reduce the chances of it happening again.