I would like to suggest a new feature for spell checking. Whenever you choose to ignore words, it will be saved within the document. I see two advantages over adding the word to dictionary:

1. The ignore is remembered wherever the document goes.
2. The ignore will not affect all of your documents.

Spell checking is still a limited field and one of the most annoying problems is wanting spell check in order to correct mistakes but having to do it again on different computers, and even so having to create artificial dictionaries to hold words you just wanted to ignore in that specific context.

I think this could be implemented as a new tag around the text for individually ignored words, and a new metafile for ignore all. Whenever the document is saved or maybe occasionally, a garbage collection is performed in that metafile for removing unreferenced items. An individually ignored item could look like this, for example:

<text:p text:style-name="P3">
    <spell:ignore>LibreOffice</spell:ignore> is an open source application and
    supports open standards for interoperability and public benefit.
</text:p>
Operating System: All
Version: 4.2.4.2 release