From an IOP news article:
Law-breaking liquid defies the rules
Monday 27 September 2004
Physicists in France have discovered a liquid that “freezes” when it is heated. Marie Plazanet and colleagues at the Université Joseph Fourier and the Institut Laue-Langevin, both in Grenoble, found that a simple solution composed of two organic compounds becomes a solid when it is heated to temperatures between 45 and 75°C, and becomes a liquid when cooled again. The team says that hydrogen bonds are responsible for this novel behaviour (M Plazanet et al. 2004 J. Chem. Phys 121 5031).