These 38 levels have no proof:
levels/single/clam/Humane/about-face.txt
levels/single/clam/Humane/bridge-burnout.txt
levels/single/clam/Humane/bucket-list.txt
levels/single/clam/Humane/nested-loops.txt
levels/single/clam/Humane/pillarity-ensues.txt
levels/single/lemforum/Cunning/needaboost.txt
levels/single/lemforum/Cunning/theob3lisk.txt
levels/single/lemforum/Cunning/wreckofthehesp.txt
levels/single/lemforum/Daunting/repeatingpatterns.txt
levels/single/lemforum/Daunting/sevenpillars.txt
levels/single/lemforum/Hopeless/thecontinuum.txt
levels/single/lemforum/Quirky/theadventureplaygr.txt
levels/single/lemforum/Simple/fourcorners.txt
levels/single/lemforum/Vicious/3111.txt
levels/single/lemforum/Vicious/chasm.txt
levels/single/lemforum/Vicious/merdemerdemerde.txt
levels/single/lemforum/Vicious/survivalofthecraft.txt
levels/single/lemforum/Vicious/thecircularruins.txt
levels/single/nepster/5-of-all/02_BigIceBlock.txt
levels/single/nepster/5-of-all/07_SmallSteps.txt
levels/single/nepster/5-of-all/12_Curiosity.txt
levels/single/nepster/Black Hole/17_VeryDerailed.txt
levels/single/nepster/Outtakes/02_Flight.txt
levels/single/nepster/Outtakes/10_LotsSmallTasks_Challenge.txt
levels/single/rubix/Nightmare/bangarangpeter.txt
levels/single/rubix/Nightmare/baywatch.txt
levels/single/rubix/Nightmare/flatlined.txt
levels/single/rubix/Nightmare/itdoesntseemtofit.txt
levels/single/rubix/Nightmare/tributetomchammer.txt
levels/single/rubix/Nightmare/undermywing.txt
levels/single/rubix/Perplexing/astoughasyouwant.txt
levels/single/rubix/Perplexing/postcardsarrived.txt
levels/single/rubix/Perplexing/pyramatic.txt
levels/single/rubix/Perplexing/quotethatsanasty.txt
levels/single/rubix/Perplexing/thestockade.txt
levels/single/rubix/Pleasant/thearmory.txt
levels/single/rubix/Pleasant/walkinginawinter.txt
levels/single/rubix/Sinister/allforoneonefora.txt

Directory coverage: 491 of 529 levels are solvable, 38 may be unsolvable.