Why do smiley emoticons change to something else at receiving end?

[AC Question]

My friends and I have just discovered that the smiley faces we thought we sent in our texts have nearly all been converted to little green martians (or whatever they are supposed to be) or even a cat face at the receiving end. Happening between Samsung Galaxy phones among others. We have compared our 'sent' texts to the 'received' texts on each others phones - not funny and could have clearly have been taken out of context - lucky we are all good friends! Any explanation or just don't trust using emoticons any more? Hoping someone can enlighten us, thanks.