GUI.Button changes position every frame

I wrote this script:

http://pastebin.com/9ExRmSMH
(Don't know why but if i try to copy and paste my script from monodevelop there, the text will be filled of chinese characters)

The problem is the button i draw when the player picks an item will change position every frame...
And i don't what's wrong....

 

Which line in the code is the button that moves?

What value is "slot.x" getting ?

 

With slot.x and slot.y i mean a and b values that i use in the void Start() to add slots to the slot class
and, the problem is that i don't know why the button moves...

 

With a Debug.Log(); i noted that for the Item[i=0], slot.x and slot.y changes every iteration...

Anyway, the button is
if(GUI.Button(new Rect(slot.x, slot.y, 50, 50),GameManager.items.icon)){

}