Maybe even entire paragraphs?
Boy, that can be embarrassing, can't it?
I know what you're going to say next: "Gee, KT, I've never seen that happen to you! How do you, what's your secret? Are you simply the most perfect writer in the whole world, who has a perfect memory and never makes mistakes?" Hah, I wish!
Here's my secret: When I'm typing text with an HTML formatting tag in it, or quotation marks, or parentheses, or any kind of delimiting character, I type both the opening and the closing characters first, then fill in the stuff in between.
For instance, to type a quote, such as "Hello, world!", instead of typing quote-H-e-l-l-o-comma-space-w-o-r-l-d-ba
In the same fashion, to italicize some text on a webpage, just type <-i->-<-/-i->-left-left-left-left and then type the text. By typing the opening and closing tags at the same time, there's no chance of forgetting the closing tag, or mismatching them, ooo or forgetting to type the slash! Very important, the slash.