Is there an error here in the number of children from Jacob's wife Leah?
Genesis 46:15 says that "These were the sons Leah bore to Jacob in Paddan Aram,[c] besides his daughter Dinah. These sons and daughters of his were thirty-three in all."
There are also issues in considering that Leah and Jacob are part of the 33 people listed.
In quoting the passage where they were listed, I will number them as they are named. The verses in this passage number 8 through 15, so I will substitute A through H in parentheses for the verse numbers so that they will not be confused with the numbering of the names in the list, which will be indicated in brackets. Er and Onan, in verse 12 (E) will not be counted, because they had died in Canaan, as the verse notes, before the Israelites went into Egypt.
(A) Now these are the names of the Israelites, Jacob and his offspring, who came to Egypt.  Reuben, Jacob's firstborn, (B) and the children of Reuben:  Hanoch,  Pallu,  Hezron, and  Carmi. (C) The children of  Simeon:  Jemuel,  Jamin,  Ohad,  Jachin,  Zohar, and  Shaul, the son of a Canaanite woman. (D) The children of  Levi:  Gershon,  Kohath, and  Merari. (E) The children of  Judah: Er, Onan,  Shelah,  Perez, and  Zerah (but Er and Onan died in the land of Canaan); and the children of Perez [already counted as number 19] were  Hezron and  Hamul. (F) The children of  Issachar:  Tola,  Puvah,  Jashub, and  Shimron. (G) The children of  Zebulun:  Sered,  Elon, and  Jahleel (H) (these are the sons of Leah, whom she bore to Jacob in Paddan-aram, together with his daughter  Dinah; in all his sons and his daughters numbered thirty-three).
There are also issues in considering... Jacob.There are other instances in Tanach where a list is described with a certain attribute (in this case, that the enumerated people were sons and daughters of Jacob), and yet that attribute is understood to apply to all but one person in the list. Per the Ibn Ezra's view (cited in Jay's answer) that Jacob himself was one of the 33 even though he's obviously not his own son, this is just one example of this phenomenon (cf. Gen. 35:26, where the description doesn't apply to Benjamin, and 36:15–16, where the description doesn't apply to Korah).