Water flowing over weirs experiences a rapidly accelerated flow region near the crest. A common type of weir crest is the circular-crested weir. Advantages of the round-crested weir shape are typically listed as a stable overflow pattern compared to a sharp-crested weir, the ease to pass floating debris, the simplicity of design, with an associated lower cost compared to the ogee crest, and a relatively large discharge capacity, compared to broad-crested and sharp-crested weirs. As part of the data sets presented by Tullis et al. (2019), the head-discharge relationship of a half-round crested weir was tested physically under carefully controlled flow conditions by the author himself. The relationship was obtained for a wide range of discharges, over two orders of magnitudes, and the results were compared to the literature on half-round crested weirs, i.e. circular weirs. The nappe was not ventilated. With increasing discharges, the nappe was initially attached to the weir's downstream wall, until some nappe detachment occurred. With a further increase in flow rate, the detached nappe re-attached at large flow rates. The transitions, i.e. both nappe detachment and re-attachment, were characterised by large instabilities, change in flow properties, sometimes loud noise, and the entire processes were subjected to some hysteresis. The finding demonstrates the complicated features of an un-ventilated round-crested weir overflow, while the importance of careful experimental procedure is discussed.