The bug
When you middle click a spawner_minecart you will get a minecart item. I am not talking about the fact that you do not get a minecart spawner item (that is MC-17653), but rather that you get a plain minecart. I would expect like it is for other blocks or entities that have no item to get nothing at all.
The reason
The reason for this is that in the net.minecraft.client.Minecraft.middleClickMouse()
method of the net.minecraft.client.Minecraft
class (MCP 1.8 names) it defaults to a normal Minecart in case no item for the corresponding Minecart type exists:
//...
else if (this.objectMouseOver.entityHit instanceof EntityMinecart)
{
EntityMinecart var12 = (EntityMinecart)this.objectMouseOver.entityHit;
switch (Minecraft.SwitchEnumMinecartType.field_178901_b[var12.func_180456_s().ordinal()])
{
case 1:
var2 = Items.furnace_minecart;
break;
case 2:
var2 = Items.chest_minecart;
break;
case 3:
var2 = Items.tnt_minecart;
break;
case 4:
var2 = Items.hopper_minecart;
break;
case 5:
var2 = Items.command_block_minecart;
break;
default:
var2 = Items.minecart;
}
}
//...
However having a method for each entity that returns the item would be probably better.
How to reproduce
Middle click a spawner_minecart
Linked issues
relates to 1
Attachments
Comments

Confirmed for 1.13.1.

Can confirm for 19w34a

Confirmed in 19w45a.

Confirmed for Minecraft 1.15 Pre-Release 3

Affects Minecraft 1.15 Pre-Release 4

Affects 1.15 Pre-release 5

Affects 20w20b

Affects 1.16 Pre-Release 2

Affects 1.16 Pre-Release 6

Affects 1.16 Pre-Release 7

Affects 1.16 rc-1

Affects 20w27a
Can confirm in 21w05b.
Can confirm in 21w07a.

Affects 1.17

What’s a spawner_Minecraft and what is the difference between that and a normal one?

Affects 1.18
Can confirm in 1.18.1.
Can confirm in 1.18.2 and 22w15a.
Can confirm in 1.19.
Can confirm in 1.19.2.

Affects 1.21.2 Pre-Release 3
Can confirm for MC 1.12.1.