The one with the difference between i.e. and e.g.

The Latin abbreviation i.e., which stands for id est, means that is, that is to say, or in other words. The letters e.g. stand for the Latin phrase exempli gratia, which means for example.

Here are a few additional points to remember about these Latin abbreviations:

*The letters within them are followed by periods and have no space between them.

* Both expressions are followed by a comma when they are being used in their functionary role (i.e., not when they are being used as nouns, as phrases being spoken about, as in many places throughout this exercise).

*In professional writing contexts, they should be used only in footnotes or parenthetically within the running text of a sentence (i.e., inside parentheses).

*A few style manuals (e.g., the Texas Law Review Manual on Usage and Style) say that these abbreviations should be italicized, but most style manuals advocate setting them in roman type.

Thanks to GetItWriteOnline.