Why might one USB port be problematic on a MacBook Pro? Background

I sometimes find that the frontmost of three ports on a 2009 MacBookPro5,2 is less capable than the other ports. 

This frontmost port with an independent bus seems: 


good enough to charge an iPhone 1 
sometimes problematic for devices that have their own power sources
sometimes problematic for simple bus-powered devices, for example a Microsoft IntelliMouse Optical.


Maybe more problematic when the laptop is booted from something other than the internal drive (Serial-ATA). 

In some problem situations, reconnecting the device to an alternative port might work around … at least temporarily. 

I read of comparable problems with: 


a 2010 MacBookPro6,2
a 2011 MacBookPro8,1.


Expectation

One port should provide up to 1100 mA at 5 V subject to the conditions given in Apple article HT4049. 

Question

Why might just one USB port of a MacBook Pro be problematic with a device that draws little power, or with a device that has an independent power supply?

Environment

MacBookPro5,2 with 8 GB memory, checked twice with Apple Service Toolkit (AST) by an Apple authorised service provider. 

To the best of my knowledge/recollection:


when I find trouble with the frontmost port, neither of the other ports has requirements exceeding 500 mA at 5 V
I have no device that requires more than 500 mA.


Rearmost port

USB High-Speed Bus:

  Host Controller Location: Built-in USB
  Host Controller Driver:   AppleUSBEHCI
  PCI Device ID:    0x7f8b00000aa9 
  PCI Revision ID:  0x7f8b000000b1 
  PCI Vendor ID:    0x7f8b000010de 
  Bus Number:   0x26 


Central port

USB High-Speed Bus:

  Host Controller Location: Built-in USB
  Host Controller Driver:   AppleUSBEHCI
  PCI Device ID:    0x7fb200000aa9 
  PCI Revision ID:  0x7fb2000000b1 
  PCI Vendor ID:    0x7fb2000010de 
  Bus Number:   0x26 


Frontmost port

USB High-Speed Bus:

  Host Controller Location: Built-in USB
  Host Controller Driver:   AppleUSBEHCI
  PCI Device ID:    0x7250505300000aa6 
  PCI Revision ID:  0x72505053000000b1 
  PCI Vendor ID:    0x72505053000010de 
  Bus Number:   0x24 


Always on bus 0x24: the built-in iSight, which requires 500 mA when not in use. 

Other buses

USB Bus:

  Host Controller Location: Built-in USB
  Host Controller Driver:   AppleUSBOHCI
  PCI Device ID:    0x7faf00000aa5 
  PCI Revision ID:  0x7faf000000b1 
  PCI Vendor ID:    0x7faf000010de 
  Bus Number:   0x04 

USB Bus:

  Host Controller Location: Built-in USB
  Host Controller Driver:   AppleUSBOHCI
  PCI Device ID:    0x7faf00000aa7 
  PCI Revision ID:  0x7faf000000b1 
  PCI Vendor ID:    0x7faf000010de 
  Bus Number:   0x06 


Links

How do you check how much power a Mac USB port can deliver?

Apple Computers and Displays: Powering peripherals through USB – article HT4049. 



Some time after posting this question I learnt that USB is amongst the focus areas for Build 12B13 of OS X 10.8.1. I'll review this question, and answers, after 10.8.1 is released.
It's difficult to tell if you have a hardware issue or this is just how things are designed.

Have you read through this article on how Apple detects the first device that requests higher power and enables that power on some, but not all ports?


Apple Computers and Displays: Powering peripherals through USB


I know you have linked to it, but the 500 mA is subject to measurement errors on the computer side - so plugging in a device can draw more current than the device says it should. Similarly, the measurement side where the Mac determines the actual power draw could be flakey or failing. This sort of thing isn't tested in the system diagnostics. You would need a test set / oscilloscope / engineering lab to really measure these currents with enough resolution to pin down exactly what is happening.

It could be just the order in which the system decides to detect a device attached. Once you have ruled that out and have a repeatable issue - you might need to have the Mac checked for service if it isn't producing power as expected. Now that you know that the order in which devices are plugged can affect things, perhaps this will help narrow down if the enhanced power is correctly available to some (changeable) subset of the ports and hubs made by Apple.

There is no easy answer other than being precise and narrowing down exactly which devices are causing errors and statistically determining/isolating whether it's the devices or the port that is out of spec and needs to be replaced.