You will get an error if Flash Fill cannot find the pattern. The first thing to check is that you're using Flash Fill close to the source data (e.g., in the adjacent cell). For instance, it won't work ...

Let’s say you have student information in a worksheet where students’ last names, first names, and ID numbers are in Column A. If you want their first name and last name to appear in Column B, you ...

When you're setting up a Microsoft Excel spreadsheet that contains date information, you'll speed your task if you let Excel do the tedious work of filling in columns of individual date entries.