I have the firewire iPod (pre-Dock). I don't know if all iPods, including Mini, suffer from this. But here is my annoyance (though I still love the thing).
It seems that my iPod suffers from improper volume level. What I mean is that the iPod only registers the volume level of a song when it is not cached. I think this also happens when using the sound check feature. Here's an example to illustrate.
Song #1 volume level gain is set at zero - so no increase/decrease
Song #2 volume level gain is set higher - let's say +3
Song #3 volume level gain is also set at zero.
While playing song #1, song #2 gets cached into the internal skip-free memory bank, as well as song #3. Once song #2 comes on, the volume level does not change to +3, it remains the same level. Now if I start song #2 over again, it adjusts the volume level as it should, but then song #3 comes along, and the volume level again does not change back to zero, it remains +3. Only when I have the song start over again, does it adjust the volume correctly.
So, to make a long story short, it seems that audio is cached into the memory bank but not any of the songs volume level adjustment attributes (the same may go for individual eq settings as well).
Of course to alleviate such problems, I've reset all my volume adjustments to zero. That just means that the certain songs (especially songs prior 1990) play at a lower volume b/c the recordings were not normalized on the original CD.