Can you tell me why this happens when I type sometimes?

Sometimes I'll type something (word processor or various internet situations) and then realize I've missed a word, so I'll put the cursor where I need it to add the word. Instead of adding what I'm typing and pushing what comes next over, the new word takes the place of what comes after it,... show more Sometimes I'll type something (word processor or various internet situations) and then realize I've missed a word, so I'll put the cursor where I need it to add the word. Instead of adding what I'm typing and pushing what comes next over, the new word takes the place of what comes after it, character by character. (This is more difficult to explain than I imagined) For example if I were to write "The dog rolled over," but wanted to add the word 'brown,' the result would be "The brownolled over." Make sense? Why does that happen and how do I fix it? Right now I just close out whatever I'm doing and reopen and it seems to work.
Update: Wow, I can't believe I never knew that...Thanks, everyone! I must be hitting it accidentally when I hit backspace...
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