After some further testing I found the item pick-up system to be generally very 
robust, and correctly (or at least predictably) handled even corner case 
situations (e.g. overlapping item labels). I did notice one irksome idiosyncrasy 
with the pickup system when item labels are displayed.

If you click the item label, a combo action is set up to move to and pick up the 
item. HOWEVER, if you don't release the mouse button immediately, move actions 
are still added, causing tux to move to the cursors current locations (wherever 
that may be) while still keeping the combo action to pickup the item. If you 
release the button in a somewhat timely manner (off of the label but still 
within ITEM_TAKE_DIST Tux will happily grab the item and you'll not even notice 
the error. However, if you hold the mouse button for slightly longer, the 
movement point gets set ever farther away. If this final point is farther than 
ITEM_TAKE_DIST Tux will stupidly walk over the item (really, check it out) and 
to wherever the target point lies.

In this manner it possible (but not guaranteed) that Tux may not pick up an item 
despite the player's click originating on the label.

As a solution I would suggest locking Tux's combo action to an item's position 
if the (currently held) click originated on that item's label and Z or X is 
active. That way just clicking again will cancel the action, but the clearly 
displayed intent of picking up an item is fulfilled without the current 
idiosyncrasies.