Parashat Vayigash introduces us to the Egyptian city of Raamses: "Yosef settled his father and his brothers, giving them holdings in the choicest part of the land of Egypt, in the region of Raamses, as Pharaoh had commanded" (47:12). As some commentators have noted, however, a verse towards the beginning of Sefer Shemot implies that this area was built only later, after Benei Yisrael's bondage began: "They set taskmasters over them to oppress them with forced labor, and they built garrison cities for Pharaoh - Pitom and Raamses" (Shemot 1:11). If the Hebrew slaves built Raamses, apparently it did not previously exist; how, then, did Yaakov and his family settle there?
For this reason, presumably, Rashi, in his commentary on that verse in Shemot, claims that "they built" actually means "they fortified." The Egyptian taskmasters had the slaves transform the two residential areas of Pitom and Raamses into well-fortified cities suitable for the storage of the country's treasuries.
The Ibn Ezra, however, explains differently. He notes a subtle difference in pronunciation between the two references to this city. In Parashat Vayigash, the silent "sheva" vowel appears underneath the letter "ayin" in the word, rendering a pronunciation of "Ramses." In Parashat Shemot, by contrast, a "chataf patach" appears underneath the "ayin," yielding a pronunciation of "Ra'amses." Based on this distinction, the Ibn Ezra claims that these are two different cities, one which Yaakov's family inhabited upon their arrival in Egypt, and another that Benei Yisrael built when they were subjected to slavery.
Rav Yitzchak Rosenblat, in his work, "Chedvat Yotzer," points out that this dispute between Rashi and Ibn Ezra will have practical ramifications in a case where the ba'al korei (the one reading the Torah in the synagogue) mispronounces the word in either of these two verses. The general principle is that when the ba'al korei misreads a word, he must repeat it only if his misreading alters the meaning of the text. According to Rashi, "Ramses" (in Parashat Vayigash) and "Ra'amses" (in Parashat Shemot) refer to the same city; as such, misreading "Ramses" as "Ra'amses" - or vice-versa - is inconsequential (though should optimally be avoided). According to the Ibn Ezra, however, confusing the two words significantly alters the meaning of the text, as we deal here with two different geographic locations; therefore, a ba'al korei who reads one instead of the other should have to reread the word correctly.