The pink box bug is created in the client when it cannot find the proper model for an item. The item is almost always portable, and this makes it difficult to pick up after dropping it. The item can still be picked up by selecting "Make nearby portables look like bags" in the Utility menu.
See also: bugs page and Misnamed Items
Pink-block (no model)
Sometimes these cannot be seen when dropped. Use utility menu "Make nearby portables look like bags" to see them. Or the pink box may become visible when you move around a little.
Placeholders:
These items have a small 'bag' model instead of a 'proper' model.