Semantics on the else statement.. ' vs. "

Because your string contains the word you're. That word contains a single quote (technically an apostrophe, but they use the same symbol) which means that as far as JavaScript is concerned your first string looked like this: '<p>Sorry, you' and then the rest of the text was considered to be outside of the string, which caused JavaScript to crash since that text does not form a valid command.

Switching to double quotes fixed the issue since the apostrophe within the string no longer closed the string early. This issue did not occur in the video because the message the instructor wrote did not contain "you're" or any other word containing an apostrophe.

You could have fixed this issue by using an "escape character" which is a special character that tells JavaScript that it should treat the next character as literal text and not assign it any special meaning. In JavaScript (and most other languages) the escape character is the backslash "\" symbol.

So if you changed your code to this:

document.write('<p>Sorry, you\'re wrong. The number was ' + randomNumber + '</p>');

It would also have worked properly. It's worth noting that the same type of issue occurs if you use double quotes to start a string that contains double quotes within it.