  We studied thermal and dynamic history effects in the vortex lattice (VL)
near the order-disorder transition in clean NbSe$_2$ single crystals. Comparing
the evolution of the effective vortex pinning and the bulk VL structure, we
observed metastable superheated and supercooled VL configurations that coexist
with a hysteretic effective pinning response due to thermal cycling of the
system. A novel scenario, governed by the interplay between (lower) elastic and
(higher) plastic energy barriers, is proposed as an explanation for our
observations: Plastic barriers, which prevent the annihilation or creation of
topological defects, require dynamic assistance to be overcome and to achieve a
stable VL at each temperature. Conversely, thermal hysteresis in the pining
response is ascribed to low energy barriers, which inhibit rearrangement within
a single VL correlation volume and are easily overcome as the relative strength
of competing interactions changes with temperature.
