The USB standard is for the host to provide 5 V to peripherals. When I measure the voltage on an attached Arduino, I have 5.78 V. Is this much too high or within specification?
The measurement that gave rise to the confusion is in measuring the 5V line on the Arduino, which I understood was directly passed through from the USB port if that was the power source.
To end confusion, the issue was caused by a multimeter whose own battery was critically low, making the device unable to function accurately. Answers and discussion below add detail.