Blocks resulting from extreme data values[]
The majority of these blocks were likely never actually meant to exist in-game, and occurred only due to the game handling extreme metadata values as it would those in usually attainable ranges, resulting in strange blocks with traits arguably analogous to garbage data.
Leaves with data value 3, 7, 11, 15[]
From the introduction of leaves in Beta 1.2 onward to the Flattening of 1.13, leaves used numerical data values within the leaves block to hold their type. As only three species existed from Beta 1.2 to release 1.1, with data values 0/1/2 (and by extension 4/5/6, 8/9/10 and 12/13/14), a data value of 3, 7, 11 or 15 would produce an undefined leaf type. This block appeared identical to conventional oak leaves, but appeared to use the spruce tint.
These were substituted out of the game with the first 1.2 release snapshot, as jungle leaves from then on occupied that space in the leaves block.
Seamless stone slabs[]
Stone slabs with metadata over the usually obtainable values would have a slightly different appearance from normal smooth stone slabs. These were progressively replaced by proper slabs.
Strange levers[]
These also had weird distorted models, and could be obtained by loading a world from 1.3 or later containing ceiling levers (0, 7, 8, 15) in 1.2.5.