could it be "shichatta"? It'll be helpful if you give us the dialogue or situation.

hi,averx.im japanese,but did you try to write "shinasatta"? we dont use "shisatta" at all.

@Osama : I think you are thinking about shisatsu suru, for observation. In which case, I think the past tense form of that is shisatsu shita/shimashita. As far as shisatta, I have no idea what that could be =\

mmm! I don`t get the meaning .... yea give it dialog and what situation you heard. for your info, those romanised Japanese learners most have misunderstandable vocabs. because they memorize and write them into the alphabet and those won`t come appear correct sounds which cause another problem cannot converse into Hiragana.

@fumiaki: u don't use しさった at all ... what for?? u mean in speech .. or when writting? :|