Overflow/saturation. I would actually speculate that it's mostly not fine and some rare instances is okay. Say I have 16 bits to represent values... It gives me a range of -32768 to +32767. If some of my values are already greater than 16384, and I scale everything up by a factor of 2 (6dB gain), then those values can no longer be represented by the 16 bits. So anything from 16384 to 32767 will all be 32767 after the gain is applied.