The most frequent mistake made in the Amidah concerns the mention of, and the request for rain, because the wording is switched bi-annually. Throughout half the year, we become accustomed to a certain wording and tend to continue even though the time has come to change.
Therefore, if a person is uncertain as to whether or not he recited the correct words, as long as thirty days have not passed since the change in wording, in the beginning of the summer or winter, we assume that he most likely erred, since he is still in the habit of using the earlier wording.
If his mistake is one that necessitate a repetition, he must go back and pray correctly. However, if thirty days already passed, when people become accustomed to the change in wording; we can assume that he most likely recited the correct wording, and he does not need to repeat the Amidah.
In order to be spared this uncertainty, following which it is necessary to go back and repeat the prayer, it is best that every person accustom himself to the new wording on the day of the change by repeating it ninety times, so that his tongue will get into the habit of reciting the new wording and he will not err. In that way, even if the person is in doubt whether or not he recited the proper wording, the assumption is that he must have recited it correctly, since he already trained his tongue ninety times to say it in accordance with the halachah. Hence, it is unnecessary to repeat his prayer. (Shulchan Aruch 114:8-9).