System Information
Windows 7 64-bit, GTX 580

Blender Version
Broken: BuildBot 2.71 revision ea07d93 (August 18th, 2014), at least all the way back to the beginning of July.

Short description of error

Run a little Python script like this:

import bpy
bpy.context.scene.frame_set(-1)

What frame does that set the scene to? You'd think it would be frame -1, but instead it's frame -2! If you enter any negative number n into frame_set(), it will set the scene to frame n-1. If you enter any non-negative number nn into frame_set, it will set the scene to frame nn.

I've been able to set the frame to -1 only by doing this:

bpy.context.scene.frame_set(-1,subframe=0.5)

But I don't think that's how it's supposed to work.

Exact steps for others to reproduce the error
This (

import bpy
#bpy.context.scene.frame_set(2) #This gives frame 2
#bpy.context.scene.frame_set(1) #This gives frame 1
#bpy.context.scene.frame_set(0) #This gives frame 0
#bpy.context.scene.frame_set(-1) #This gives frame -2
#bpy.context.scene.frame_set(-2) #This gives frame -3
#bpy.context.scene.frame_set(-1,subframe=0.5) #This gives frame -1

Uncomment any one of the lines and verify that the frame that the scene was set to matches the end-of-line comment's predictions.