*The leading tone, alot of minor scales (or modes) don't have a leading tone. So the seventh of the scale is often sharp, when it resolves to the root note.
* At a passing tone. example would be going A to A# to B. Sometimes called connect tones.
* When you use neighboring tones, usually the lower (hardly ever the upper N.T). Example is in A major, you may get D to C# to D.
* The use of suspensions when they don't occur naturally. Take a suspension 4th, which is the note a perfect fourth above the root. Say the root of the chord is F and you in the key of C major. B occurs naturally but it's not a perfect forth, so B is sometimes flattened. (Bb)
* Borrowing from modes. Say the song is in the key of C major, you could use a modal scale for the melody. C lydian for example. There is only one altered tone F to F#. This effect is to temporary change the key, but since they are so closely related. The next note you would play would exist in the originally key.
* The use of the diminished 7th chord right before a modulation. In C major the dim7 is Bdim7 (B D F Ab). Ab is the altered tone.
There are many others, usually from the use of altered or chromatic harmony. Then there are just ones that are way out there.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.
