Hi,
According to it's description, $subject "Inserts a toolbar item before the Quit button, or after the previous plugin toolbar item. A separator is added on the first call to this function [...]".
In reality, they are added like this: b c d e f | a Quit.
Looking at $subject source, it's obvious that the 2nd and subsequent items are inserted before the separator created with the 1st item.
There are two ways to fix this:
Keep item counter, and insert the 2nd+ items at separator_position + counter + 1; we must count the removals as well.
Always insert before Quit, which will reduce plugin_add_toolbar_item by about 1/3.
Practically the two are identical, but the latter is much simpler. If you agree to it, I'll write a patch.
Currently $subject is used by 4 plugins; they all add a single item.