Why did PerfectIt miss one instance of an error but find all of the others?

It's important to remember that PerfectIt is a tool to assist in editing, not a replacement for a human editor. PerfectIt is not intelligent; it does not understand the subtleties of language; and it will miss some errors. Always check through your document in addition to using PerfectIt.

Some errors are particularly hard for PerfectIt to find. PerfectIt is especially likely to miss an instance of an error if:

• Part of the document is set to not check spelling or grammar. Word has an option for text called 'Do not check spelling or grammar' (it's a checkbox under 'Language', then 'Set language'). If PerfectIt is set to exclude sections marked that way, and if that box is ticked, that section will be skipped by PerfectIt.
• You're working with 'tracked changes' in PerfectIt but have not hidden them (or have not enabled Revisions Mode in PerfectIt 5). To enable Revisions Mode in PerfectIt 5, click on PerfectIt's 'Tests' menu and then select 'Tests and Options'. Then click the 'Revisions Mode' tab and tick 'Use revisions mode with tracked changes'.
• You're using PerfectIt's check for Phrases in Capitals with paragraphs that have an internal heading (i.e. the first few words are in bold/capitals followed by a colon).
• While using PerfectIt's check for Phrases in Capitals you make a lot of changes using PerfectIt. Since the patterns that PerfectIt checks depend on all the words around a capitalized phrase, you may want to run the check twice to make sure that PerfectIt picks up as much as possible.
• You're working with PerfectIt's checks for abbreviations and the abbreviation appears only in tables (rather than in a sentence).
• You're using PerfectIt's check of Numbers in Sentences. PerfectIt is designed to only identify the locations that are most likely to be errors, and may skip others such as measurements or other uses that are usually intentional.