Sokuon, the little つ that usually lengthens a consonant (makes it geminate), usually only appears in front of さ, た, か, and ぱ row characters. These are only unvoiced consonants. As far as I know, this is always the case for Japanese words. Loanwords are funny with voiced consonants sometimes, like big: ビッグ.
I read in a few places that there are applications of sokuon for loanwords from other languages, e.g. r-row hiragana for rolling r in Italian. One especially interesting case is the Pokemon Cramorant that is ウッウ, so a sokuon before a vowel, making it a bit like a glottal stop.
Did you come across more interesting or non-standard sokuon occurrences?
My understanding (although I’m very much a beginner) is that っ always represents a glottal stop, but a glottal stop in front of a consonant typically turns into a geminate. So it’s not that the usage of the sokuon has changed; it’s that the vocal effect changes depending on the surrounding sounds.
Can anyone else confirm/deny this? My linguistics 101 course was way too long ago.