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 on nether portal blocks, but in previous versions it was possible to place them on any block.

Obtaining

Sign on a Non-opaque Block

From Infdev 20100607 (when signs were added) to Alpha v1.0.1, it was possible to place signs on any block, with the disadvantage that when updated, the sign would break.

Floating Sign

From Infdev 20100607 (when signs were added) to Alpha v1.0.1, 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 v1.0.1_01

Sign placed on Nether Portal Blocks

From Alpha v1.2.0 with the addition of the Nether in Beta 1.5_01, you could place signs on nether portals. To do this, simply right-click the portal block while holding a sign. From Beta 1.6 onward, you can no longer place signs on portals.

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  Sign Placed on nether portal block in Beta 1.5_01

References