Designing and completing clinical intervention trials can be challenging. Many aspects must be considered to ensure that patients who fulfill the inclusion criteria for the intervention are identified and recruited effectively. The aim of this paper was to disseminate the results of a cancelled trial and present unpredictable barriers met underway, so future researchers can learn from these. The trial examined perioperative analgesic and anxiolytic effects of melatonin. It was registered at https://clinicaltrials.gov/ (NCT02386319) and a study protocol was published a priori. Participants were recruited from the plastic surgery ward of a Danish private hospital. The intended sample size of the trial was 72 patients based on power calculations of the outcome measures. During the six-month recruitment period, six patients were included, with only three completing the trial. Unpredictable barriers were poor communication between investigators and facility staff, lack of access to booking and operation schedules at the recruitment facility, the patient group being unwilling to participate, and the timing of recruitment conversations being unsuited as patients often did not have time to talk to the investigators. Too few data were collected to make any meaningful statistical analyses. Our trial was cancelled prematurely because of unpredictable barriers after commencing recruitment. Considering these barriers when designing a clinical trial may help future researchers avoid cancelling trials. Transparency of research is important and even prematurely cancelled trials should publish their findings.