That is an excellent question, and it has been raised by the Pnei Yehoshua. Let us have a look at the various answers that have been suggested for your question, and if there is still a need - we might suggest an approach of our own.

1) Zichron Yisachar (Lewental, 1933, on Mishnayos Moed) suggests that the Mishnah means that if we allow people to carry to Reshus ha'Rabim shortly before Shabbos starts, one Shabbos the person will accidentally carry after Shabbos has already begun.

The problem with this is that if so, the Gezeirah is not because one might 'forget' the tailor's needle, but rather one might 'mistake', or miscalculate when Shabbos begins. Not only does that not fit into the words of the Mishnah, it is contradicted by the Gemara which tells us that one may walk out with Tefillin shortly before Shabbos since a person must never forget the Tefillin when he is wearing them. This would not help if the problem was one of miscalculating when Shabbos begins, as pointed out by Tosfos ha'Rosh (11a DH Shema Yishkach; see also Tosfos on this Mishnah).

2) The Zichron Yisachar offers another suggestion. The Mishnah may mean to prohibit taking a needle to a Chatzer (which is a Reshus ha'Yachid, like a house), lest one take it out to a Reshus ha'Rabim .

The problems with this are:

(a) This is not the normal meaning of "Yetzei" in Maseches Shabbos, and it is quite clear from the Beraisa in the Gemara that it is not the meaning of Yetzei here either. The Beraisa uses the exact same wording as the Mishnah - לא יצא החייט במחטו - and rules that if one does so on Shabbos he is Chayav Chatas. Clearly, the needle is being taken into a Reshus ha'Rabim.

(b) Besides, prohibiting a person from carrying even in his own Chatzer before Shabbos would be a tremendous Chumra, not mentioned in Poskim.

3) Chidushei Rav Binyamin Zev Zinger (a Rav in Europe ~100 yrs. ago) suggests another approach to answer your question. The Mishnah is assuming that the person who takes a needle into Reshus ha'Rabim shortly before Shabbos - will return it to Reshus ha'Yachid before Shabbos. (This is reasonable to assume. If the person does not plan on returning to Reshus ha'Yachid - he must have intended to carry the needle in Reshus ha'Rabim on Shabbos, which constitutes an Isur d'Oraisa. A Rasha who does that will not be interested in hearing Gezeiros of the Rabanan, so the Mishnah cannot be addressing him.) If so, the Mishnah may be concerned that after returning to Reshus ha'Yachid , the person may forget the needle that is in his hand and take it back out to Reshus ha'Rabim.

The problems with this are:

(a) the Mishnah should have added a word - שמא ישכח ויחזור ויצא, perhaps he will go out again .

(b) If even after the person returns to his home we are concerned that he will take the needle out again - what difference does it make if he originally took it out and returned home, or if he never left his house. In either case we should be equally concerned concerned that the person will forget and take the needle out.

(c) In addition, is we are concerned that a person sitting at home might walk outside on Shabbos if he is holding a needle in his hand - then the Gezeirah should apply throughout Shabbos, not just shortly before Shabbos begins! It should never be permitted for the tailor to take his needle in hand on Shabbos lest he forget and walk out with it.

4) The Pnei Yehoshua himself answers that the word "v'Yetzei" does not mean "he will take it out ". It means "he will transgress the prohibition of Hotza'ah on Shabbos". That is, he may walk with the needle 4 Amos in Reshus ha'Rabim, or bring it into Reshus ha'Yachid.

It turns out that the Pnei Yehoshua was Mechaven to the words of a Talmid of Rashi, the Machzor Vitri, who asks your question and gives just that answer (in Hilchos Lulav #3). This also seems to be the interpretation of the Rambam (end of ch. 19 of Hil. Shabbos) who changes the words of the Mishnah and writes "שמא ישכח ויוציא" - perhaps he will do Hotza'ah .

This is a satisfactory answer, but it also has its problems.

(a) The word 'v'Yetzei' certainly does not imply Hotza'ah. The Mishnah should have said, as the Rambam did, 'v'Yotzi' (with an extra Vav and Yud). Although the Rambam indeed uses that word when citing the Mishnah, it is quite clear that he is explaining, rather than quoting verbatim, the words of the Mishnah. There is no record anywhere of a Girsa that has the word 'Yotzi' in the Mishnah - not in the Rambam's version of the Mishnah (as published by R. Kapach) or in any of the other copies we have of the Mishnah and Gemara.

(b) The Mishnah would still be leaving out a word. It should have said שמא ישכח ויוציא משתחשך - perhaps he will take it out after sundown - since until now we were discussing taking it out before sundown. Rashi, in fact, seems to sense that the word is missing, and he adds משתחשך to the words of the Mishnah when citing the Mishnah on 9b (DH Ad she'Yispalel) and when commenting on the Mishnah here (11a).

5) So since there is still room for improvement, I will offer my own explanation for our Mishnah.

In Beitzah 8a, when the Mishnah says שאפר כירה מוכן הוא, the Gemara explains that it means ואפר כירה מוכן הוא; the שי"ן is supposed to be read as a וי"ו. This transposition can be made in reverse as well, as pointed out by many, such as the Tosfos Yom Tov (Bava Basra 2:8; Nidah 4:1) and Margenisa Tava (Sefer ha'Mitzvos, Shoresh 5 section 1, citing one of the early Medakdekim; his example is דבר אל בני ישראל ויקחו לי תרומה which is read like שיקחו לי תרומה).

With this in mind, the Mishnah may mean שמא ישכח שיצא - lest he forget that he went out (with Kamatz's under the Yud and Tzadi). That is, the Mishnah is telling us that the tailor may not go out with his needle before sunset, lest he forget that he went out [i.e. he went out with his needle ; the Mishnah is relying on the words "with his needle" written in the beginning of the Mishnah].

The Mishnah is now perfectly clear and does not leave out any critical words.