Today while praticing guitar I've played the following notes on the fretboard, one after another:
E F G G# A B C D# E
It didn't sound wrong, in fact, it sounded like Harmonic Minor with something else. Nevertheless I could not find anything about what this pattern could be.
Anyway, is there any kind of scale that contains eight different notes? If the answer is yes, what is the purpose of these "aditional" notes?
The scale you mentioned appears to be a double harmonic with a flat 3rd added. This flat 3rd, in combination with the flat 2nd of the double harmonic, can give it a kind of phrygian mode feel, but then there's the leading tone too, so it's not completely phrygian mode.
Ely was right in that octatonic scales contain 8 notes. However, not all octatonic scales are structured the same way. The specific one described in that answer can be called a diminished scale, since it includes the eight notes of two adjacent fully diminished seventh chords. This scale is also neat in that it is a symmetric scale, which allows the "center" to be transposed, or shifted around, while still keeping the intervals in the same order.
Another example of an octatonic scale would be the bebop scale, which is basically the dominant scale (minor 7th) plus a major 7th. This is interesting, since it has the capability to have a dominant feel as well as a leading tone.
Some wikipedia links if you want to read more about this stuff:
(I can't post more than two links, but also check out the page titled "Modes of limited transposition" for some awesomeness about symmetric scales.)
The octatonic scale contains 8 different notes. It follows an alternating whole/half step pattern. So: C, Db, Eb, Fb, Gb, Abb, Bbb, Cbb C I think.
Typical 8 note scales are the bebop scale which has the #7, and the diminished scale.
Barry Harris (1929-2021) taught the following eight-note scales, which can be harmonised by two chords (and their inversions) only; the tonic chord and the second chord which is the leading-note diminished seventh chord.
C D Eb F G Ab A B, chords Cm6 and Bdim7 (minor sixth diminished scale, bebop melodic minor scale
C D E F G Ab A B, chords C6 and Bdim7 (major sixth diminished scale, bebop major scale)
C D E F Gb Ab Bb B, chords C7b5 and Bdim7 (seventh flat 5 diminished scale)
C D E F G Ab Bb B, chords C7 and Bdim7 (dominant seventh diminished scale)
C D Eb F Gb Ab A B, chords Cdim7 and Bdim7 (whole step/half-step diminished scale)
These scales have also another thing in common - they are made from whole tones and half tones only. In other words they do not contain augmented second, i.e. there is no "three-halfsteps-space" between the neighbour notes (as it is e.g. in the A harmonic minor seven-note scale between F and G#).
If we apply this "no augmented second" constraint to the selection process, there are five more eight-note scales made from whole tones and half tones only. They are again harmonised by two chords only, however the second one on the leading note is not the diminished seventh chord.
C D E F# G# A Bb B, chords C7#5 and Bm7
C D E F# G G# A B, chords C6 and Bm6
C D E F G A Bb B, chords C7 and Bm7b5 (bebop dominant scale)
C D E F# G A Bb B, chords C7 and Bm7
C D E F# G G# Bb B, chords C7 and Bm6
