Some words have more than one correct spelling. American, British, Australian and Canadian English all have their own preferences. Even within those, there can be multiple spellings. For example, in the UK 'realise' is often preferred. However, 'realize' has been used in British-English for centuries and is preferred in the Oxford English Dictionary. However, no matter which spelling is preferred, one thing is always wrong: you mustn't use two different spellings in the same document.