BACKGROUND Pediatric condition falsification (PCF) is a rare form of child abuse in which a caregiver fabricates or induces illness in the child. The diagnosis is difficult and controversial and can easily include false positives. CASE REPORT A boy, 3.18 kg birthweight (P25 curve), lost weight between age 56 to120 days. Cow milk allergy was suspected, feeding was changed to elementary formula, and he started catch-up weight growth while remaining significantly underweight. His pediatrician continuously interpreted his low weight as insufficient growth, despite prescribing 3 times the normal caloric intake, concluded that the mother purposely malnourished her son, diagnosed PCF, and the boy was separated from his family (days 502-755 of age). PCF was confirmed by 2 other pediatricians and 3 child protection physicians and was supported by 4 child protection agencies and 6 judges. However, proper analysis of the weight growth (kg/year) from the weight curve showed a normal weight gain. Beyond 120 days of age, weight gain at home was significantly above normal (during 347-489 days: 6.2 versus 3 kg/year of the P50). He reached P25 again at around 516 days. CONCLUSIONS The question "How could so many physicians misjudge weight gain?" has scientific and sociologic aspects. Scientifically, low weight was wrongly interpreted as insufficient weight growth, requiring that physicians learn how to assess weight gain from weight curves. Sociologically, physicians seem to follow a diagnosis made by a colleague without proper evaluation. Arguments provided by the parents against this diagnosis seemed to be neglected. Confirmation bias occurs when any information against PCF is disregarded.