Mac OS X Mavericks has an autocorrect function that can be very anoying. You can disable it in the system preferences completely.
Just open system preferences, klick on keyboard an select the "Text"-tab. There you can uncheck "Correct spelling automatically".
If you don't want to disable this feature completely, you can disable it for single applications as well. Open an application with text input like evernote and select "Edit" in the menu-bar. You can disable autocorrection In the submenu "Spelling and grammar"