Having mismatched tags may or may not affect browsers atm but the code is invalid according to the w3c validator.
Mismatched tags might cause web pages to break in future browser releases though.
If you use a reputable IDE like Netbeans
or something similar, it indents all your code for you which makes it much easier to spot mismatched tags. Also if you click on a tag, it will highlight its matching tag if it exists.