The entry about "transfinite number" in EB claims that aleph-one is the cardinality of the real numbers. This is in fact neither provable nor disprovable; see cardinality and continuum hypothesis for the full story.
In "number game" under "logical paradoxes" in EB it is claimed that the statement "All Cretans are liars", if uttered by a Cretan, is self-contradictory. This is false; see Epimenides paradox for the correction. AxelBoldt sent a letter to the math editor of EB to point this out and received an answer three months later saying that his "sources" disagreed and considered the text to be correct.
In "NP-complete problem" you can find the statement
Under "refractive index" in EB, the definition of the refractive index does not clarify the distinction between phase velocity and signal velocity; it is stated that the velocity of x-rays in glass is higher than the velocity of x-rays in vacuum. This is true for the [phase velocity]?, but the speed with which information can be transmitted is not higher in glass than in vacuum.
EB claims that the cardinal number aleph-0 is commonly added to the class of real numbers; this is not common at all. See real number.
The EB article about "Olbers' paradox" offers a resolution involving average lifetimes of stars which does not make sense. See Olber's paradox.
"Big O" is a common notation in computer science and mathematics, used to compare the speed of growth of functions (also called "Landau's symbol"); it is also the name of a Japanese Anime show. Neither of which can be found in EB. See Big O and Big O anime.
EB claims in its leap year article that years divisible by 4000 may be non-leap years. This is in fact not an official rule and would not increase the calendar's accuracy. See leap year.
The Fast Fourier Transform, a fast algorithm to compute the discrete Fourier transform, has been called one of the most important algorithms ever. Neither the Fast Fourier Transform nor the discrete Fourier transform are mentioned in EB.