If you have accidentally saved a word into the T9 dictionary (ie, "IS" comes up with "47" or "Get" comes up with "HDU", you can teach the dictionary to revert back to the original word.
What you have to do is..... create a new text message, enter the text that is annoying, (ie, "IS"). When you type the word and it comes up with the annoying one, select the word that you wanted and enter a space, and do it again and again - you can do this in the same text message - it can take up to 20 goes but eventually, the dictionary recognises that you actually wanted "IS".