Underlying patterns and factors behind suicides of patients in treatment are still unclear and there is a pressing need for more studies to address this knowledge gap. We analysed 278 cases of suicide reported to The Norwegian System of Patient Injury Compensation, drawing on anonymised data, i.e., age group, gender, diagnostic category, type of treatment provided, inpatient vs. outpatient status, type of treatment facility, and expert assessments of medical errors. The data originated from compensation claim forms, expert assessments, and medical records. Chi-square tests for independence, multinominal logistic regression, and Bayes factors for independence were used to analyse whether the age group, gender, diagnostic category, inpatient/outpatient status, type of institution, and type of treatment received by patients that had died by suicide were associated with different types of medical errors. Patients who received medication tended to be proportionally more exposed to an insufficient level of observation. Those who received medication and psychotherapy tended to be proportionally more exposed to inadequate treatment, including inadequate medication. Inpatients were more likely to be exposed to inappropriate diagnostics and inadequate treatment and follow up while outpatients to insufficient level of observation and inadequate suicide risk assessment. We conclude that the patients who had received medication as their main treatment tended to have been insufficiently observed, while patients who had received psychotherapy and medication tended to have been provided insufficient treatment, including inadequate medication. These observations may be used as learning points for the suicide prevention of patients in treatment in Norwegian psychiatric services.