Here's the problem:
The "global gain" field that mp3gain adjusts is an 8-bit unsigned integer, so the possible values are 0 to 255.

MOST mp3 files (in fact, ALL the mp3 files I've examined so far) don't go over 230. So there's plenty of headroom on top-- you can increase the gain by 37dB (multiplying the amplitude by 76) without a problem.

The problem is at the bottom of the range. Some encoders create frames with 0 as the global gain for silent frames. What happens when you _lower_ the global gain by 1? Well, in the past, mp3gain always simply wrapped the result up to 255. That way, if you lowered the gain by any amount and then raised it by the same amount, the mp3 would always be _exactly_ the same.

There are a few encoders out there, unfortunately, that create 0-gain frames with other audio data in the frame. As long as the global gain is 0, you'll never hear the data. But if you lower the gain on such a file, the global gain is suddenly _huge_. If you play this modified file, there might be a brief, very loud blip.

So now the default behavior of mp3gain is to _not_ wrap gain changes. In other words,
1) If the gain change would make a frame's global gain drop below 0, then the global gain is set to 0.
2) If the gain change would make a frame's global gain grow above 255, then the global gain is set to 255.
3) If a frame's global gain field is already 0, it is not changed, even if the gain change is a positive number

To use the original "wrapping" behavior, use the "/w" switch.

If n > 0
   For each g[i]
      If g[i] + n > 255
         n = 255-g[i]
Else If n < 0
   For each g[i]
      If g[i] - n < 0
         n = 0-g[i]
Print a warning in case n had to be changed.