Java Edition:Invalid Block Sign

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Signs can normally only be placed on solid and semi-solid blocks, with some exceptions like placing signs in nether portal blocks, but in previous versions it was possible to place them on any block.

Obtaining

Sign on a non-opaque block

From Inf-20100607 (When signs were added) to Alpha 1.0.1, it was possible to place signs on any block, with the disadvantage that when updated, the sign would break.

Floating sign

From Inf-20100607 (when Signs were added) to Alpha 1.0.1, it was possible to create floating Signs by placing them on a block that could be mined instantly and breaking it at the same time the Sign was placed.

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  Sapling before placing the sign
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  Sapling after placing the sign
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  Floating sign in Alpha 1.0.1_01

Sign placed in Nether Portal Blocks

From Alpha v1.2 with the addition of the Nether to Beta 1.5_01, you could place signs at nether portals. To do this, simply right-click the portal while holding a sign. From Beta 1.6 onward, you can no longer place signs on portals.

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  Sign Placed In Nether portal block in Beta 1.5_01