Not really sure whether to class this as a bug, feature request or nit pick so sorry if this is the wrong place. It seems a bit like all three.
The issue is that the fade in/out fields in Preferences > Media Item Defaults accept values less than 0.001 but only display three decimal places.
While seeming like a trivial issue, there's a bit more to it: If you do enter a value less than 0.001, the next time you modify anything else in Preferences this default fade length in REAPER.ini is overridden by the 0.000 that is shown in the UI.
It would be great if these fields could either display more than 3 decimal places or at least not override the value.
I guess there are other places where this happens. For example, the item properties window likewise accepts fade values of < 0.001 but only displays 0.000. This doesn't have the same overriding issue luckily though.