This is the single most useful keyboard shortcut I’ve ever found, but it seems no-one knows about it; at least, if they do, they didn’t tell me at any point in the first 33 years of my computer career.
If you copy (Ctrl-C) from a web-page, or a Word/OpenOffice document, or any other source that has formatting, then when you paste the copied material into a document that supports formatting (such as another Word/OpenOffice document or a WordPress post), the formatting — or at least, a broken attempt at it — will be pasted in. That usually means that you get fonts you didn’t want, that are inconsistent with the rest of the document. I imagine this is the cause of most of the horrible Font Soup you see in too many MS-Word documents.
This is a proper pain if, to pick a purely hypothetical example, you’re putting together a book based on your own blog-posts.
So instead, use paste-without-formatting. In OpenOffice (and its derivatives: NeoOffice, LibreOffice, etc.) this is on the Edit menu as Paste Special…, but you can use the shortcut Shift-Ctrl-V (or Apple-Ctrl-V on a Mac) to invoke it. It pops up a small menu of paste styles: double-click on Unformatted Text and you’re done.
It turns out that although there is no corresponding menu-item in the WordPress composer, the keyboard shortcut works — in fact, it works even better than it does in OpenOffice, as it doesn’t bother with the menu and just does the paste as unformatted text.
Shift-Ctrl-V: it’s your friend.