There was a lengthy thread last year on the underpowered RCT topic, albeit, it also segued into some discussions about interim monitoring and related topics:
In the course of the discussion, in one of my replies, I referenced the following 2013 paper by Button et al:
Power failure: why small sample size undermines the reliability of neuroscience
On the skill related issues that @trumanfrancis raises above, that is a key reason why in many studies, especially for implantable devices in IDE studies, you will see that for each of the one or more study sites/PIs, there will be an initial series of “run-in” subjects of some modest number, to get over the learning curve issue that may be present and bias outcomes for early subjects.