Abstract
It is well known that a center of a given circle cannot be constructed using only a straightedge and that this was proven by David Hilbert. Still it is not so clear what kind of object is proven to be non-existing. We analyze different attempts to define a geometric construction appearing in the literature and observe that none of them is really satisfactory and that Hilbert's proof needs to be corrected (as noted by Akopyan and Fedorov).