The quote is talking about code that is supposed to find the ceiling of a number, and does so by adding 0.5 and then rounding to the nearest value (breaking tie

The quote is talking about code that is supposed to find the ceiling of a number, and does so by adding 0.5 and then rounding to the nearest value (breaking ties by rounding towards even). If you have a very small but non-zero positive number, it gets rounded to 0.5 after the addition, and then the second rounding incorrectly rounds toward 0 instead of 1. The result is that you have a value x such that x > ceil(x), which shouldn't ever happen.