Java Edition removed blocks/Data variants

Blocks in flower pots that should not be in flower pots
From 1.7 onward, as the amount of blocks that could be placed within pots exceeded 15, conventional 4-bit metadata values could not be used to store the contents of a flower pot, and as such a block entity needed to be used instead. This allowed any block (or indeed item) to be placed within a flower pot, but few of these would actually render with the block inside.

In 14w17a, almost certainly due to changes in how blocks render with them being changed to use block models rather than hardcoded models, these odd pot variants simply rendered as empty pots with no further interesting quirks, aside from the fact that their "invalid" contents could be retrieved from the pot via breaking or later right clicking it. They were completely removed in 17w47a, which split the flower pot block up into an individual block ID per potted object and scrapped the tile entity.

13w36a
The first version to make flower pots use a block entity, the fact that flower pots could hold ferns bled over somewhat into its other damage values, allowing shrubs and grass to exist in pots as well. Also, cobwebs would render if placed in a flower pot.

14w06a
In this version, a large amount of blocks now render if placed in flower pots via commands. In order to place these with commands, use the following command, with ITEM and DATA substituted with the appropriate values from Java Edition data value/Pre-flattening/Block IDs as necessary:

/setblock ~ ~ ~ minecraft:flower_pot 0 destroy {Item:ITEM,Data:DATA}

Only blocks that are completely unique in their rendering are shown here; some visually identical blocks do exist that are not listed here separately, such as the early stages of both carrots and potatoes, or potted pumpkins and jack o'lanterns.

The blocks always appear as a cross shape based on their bottom texture. Blocks with translucency appear either completely opaque or, in the case of stained glass, most pixels are invisible.

Some blocks appear tinted by biome if their normal variants usually would be. As leaves were tinted as per their numerical data values, setting the number prior to destroy to 1 for specifically potted leaves causes them to be tinted with the spruce color, and setting it to 2 gives the birch color. 0 or 3 results in them being tinted per biome like oak and jungle leaves are.

14w06b
In 14w06b, likely due to the fix for, an additional 57 visually distinct potted objects could be placed:

14w07a
14w07a introduced the iron trapdoor, which rendered in a flower pot.