This case report highlights the risks that may be associated with amoebae in the water of a dental unit. A woman with contact lenses visited her dentist for replacement of a bridge. During the treatment, a stream of water was directed from the handpiece into her right eye. Because of subsequent pain in the eye, the patient consulted several ophthalmologists, who discovered abrasive lesions of the cornea and inflammation. Despite antibacterial and anti-inflammatory treatments, the patient's visual acuity declined gradually over a period of several days. A microbiological examination nearly 2 months later revealed amoebae in corneal samples. A lawsuit against the dentist was initiated. Although a causal relation with the dental treatment was rejected by the judge in this case, high numbers of amoebae in the water of dental units can present a risk if a patient with pre-existing corneal lesions is splashed. According to the precautionary principle, complete evidence of risk does not have to exist to institute measures to protect individuals and society from that risk. This case reinforces the importance of having patients wear safety glasses during dental treatments and of dental personnel draining the waterlines of dental units, as recommended by the Canadian Dental Association.