Artificial pancreas systems (APS) are supposed to minimize the workload of managing type 1 diabetes mellitus for patients, but even APS users are still supposed to count carbohydrates and announce meals to achieve the best control. Here, we present the effect of intentional underestimation of food carbohydrate content and other diet-related negligence on glycaemic patterns in patients utilizing a loop-based APS. Since the patient started using the “Loop”, his glycaemic control has been satisfactory (glycated haemoglobin: 6.7% (50 mmol/mol) to 6.4% (46 mmol/mol). The problems with glycaemic control occurred when the patient went on holiday. During this period, the patient deliberately underestimated his carbohydrate meal content announced to the APS. He also used the “override” function (set to 70–80% of normal insulin requirements), which reduced the amount of insulin delivered. The patient tended to consume more alcohol and routinely consumed high glycaemic index products during this period of time. The food bolus was delivered either just before or even during meals. No loop system settings were modified. An intentional underestimation of food carbohydrate content led to the deterioration of glycaemic control in the user of the loop-based APS; however, one can speculate that with some dietary modifications (e.g. limiting carbohydrate intake) the patient could achieve better glycaemic patterns.