observed event
If I use a cactus to turn a minecart into an item, its position changes.
This happens when I bend sideways on the rails just before hitting the cactus.
Minecart is observing the following patterns: being sucked into the hopper; hitting the cactus and being lost; and slipping through the cactus.
expected event
In the current version, the only way to convert minecarts into items is by using cactus.
Since minecarts are supposed to turn into items the moment they touch a cactus, we must collect them in the hopper in front of the cactus, regardless of their condition.
The big problem is that minecarts are lost when they touch cactus.
Minecarts have slightly more than 4 health, so they take 4 ticks to break when touching a cactus. When the break, the minecart item spawns at the exact center of the minecart position. These mechanics work consistently, make sense intuitively, and have always worked this way. The issue described in the report is a build design issue, not a bug in the game.
In the build shown in the screenshot, the curved rail puts the minecart inside of the cactus, as it would with any block. The minecart goes more than halfway through the cactus before it breaks. Therefore, when the item spawns, it spawns toward the outside of the cactus and then moves out of the cactus block as it would with any block. Items have 5 health, and it seems that items either take 5 ticks to die when touching a cactus, or they have immunity for a few ticks, and that is why the item is not destroyed. It is possible to catch the item in a hopper or hopper minecart in a variety of ways with a curved rail setup like this. Here is one example:
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