The Lost Arts are an exhibition of hypothetical paintings belonging to the Great Artists of the past, including such members as Michelangelo, van Gogh, Warhol, The Guild of Unfortunate Painters and Rob. The exhibition typically reoccurs thrice every trimester of the previous year (or so it is said), based on the ticking of a temporally divorced atomic clock with atoms borrowed from a universe thrice-removed from ours (or so it is said).
Alternatively, if you have come here fresh from reading about the Abuti, you could be thinking of the Lost Arts which are so-called because society at large has either misplaced their knowledge of certain ancient technologies or mislead themselves as to what use the inscrutable machinery had. It is entirely possible part of the blame could be laid at the collective feet of the HIP.
The "rube-goldberg machine" is an infamous example typically trotted out for examination. Its internal workings as described in sources throughout the ages would almost have to leave the ordinary dabbler in the sciences puzzled and deeply conflicted.
Other arts thought lost are that of oiling a creaky typewriter at three in the morning, or trawling the dangerous if whimsical recesses of SEUSSNETS.