In high-temperature superconductors, magnetic field lines penetrate the samples through vortices arranged in an Abrikosov vortex lattice. In a magnetic field Hm(T ) below the upper critical field Hc2(T ) that destroys bulk superconductivity, the vortex lattice melts to a liquid vortex state, in which each vortex line must be `pinned' individually to prevent dissipation. Linear and planar defects have been found to be effective for pinning the vortex liquid because they trap an entire vortex within a single extended defect. However, up to now it is not known how far into the liquid state this pinning process can be effective. Here, we show that there is a universal magnetic field line Hl(T ) between Hm(T ) and Hc2(T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field Hl(T ) sets an upper limit to the irreversibility line Hirr(T ) marking the onset of dissipation. For that reason it determines a new magnetic-field-temperature region in which a superconductor can remain useful.