Here is the PowerShell script that shows the bug:
(on the other hand, it is not even needed, just double check the driver code of these methods)

Add-Type -Path MongoDB.Driver.dll

1. DOCS: http://www.mongodb.org/display/DOCS/Updating

1. Output:
   Unknown macro: { "$pop" }
2. Expected, see DOCS:
   Unknown macro: { $pop }

   $PopFirst = [MongoDB.Driver.Builders.Update]::PopFirst("name")
   $PopFirst.ToString()

1. Output:
   Unknown macro: { "$pop" }
2. Expected, see DOCS:
   Unknown macro: { $pop }

   $PopLast = [MongoDB.Driver.Builders.Update]::PopLast("name")
   $PopLast.ToString()

Note: I have also tried some real example but the code is not so easy to share. Yes, these function do the opposite job, as if their bodies are swapped by mistake.