The gemara (Yoma 73b) asks how the U'rim v"Tumin could answer questions of Bnei Yisrael when there was no letter tzad"i in the names of the shevatim inscribed upon it (Rashi: if the question asked required an answer of "tatzil" [save], how could it be given?). The gemara answers that in addition to the names of the shevatim there was also inscribed "Avraham, Yitzchak, v'Ya'akov". The gemara further asks that the letter te"is was also missing, and the gemara answers that the words "Shivtei Yeshurun" were inscribed as well.
The Chasam Sofer has an elaborate pilpul (quoted in many likut seforim on the parsha) to explain why the gemara first was bothered by the missing letter tzad"i and only afterwards by the missing te"is. Following the order of the aleph-beis, one should first notice the missing tei"s before the missing tzad"i!
I'll leave it to you to look up the Chasam Sofer and will just present my own answer here. The gemara's shakla v'terya does not follow the sequential order of the aleph-bes; rather, it follows the degree of usefulness of the letter. The gemara first asked about the missing tzad"i because tzad"i seems necessary for meaningful responses. I would suggest that Rashi deliberately gave us an example of the use of tzad"i in the world "tatzil" and no example of the use of tei"s to emphasize this point. We could of course think of our own example of the use of tzad"i without Rashi. Rashi is not just providing an arbitrary illustration of the letter's use, but Rashi is telling us that the usefulness or need for the letter is what motivated the gemara's question. Only after this issue was addressed does the gemara move to ask about the missing letter tei"s, a letter far less useful, as evidenced by Rashi offering no example of a response which would be impossible to formulate without a tei"s, but a letter which the gemara at this point still things should have been available if needed.