Not closing <img /> or <meta /> tags.
You can usually get away with not closing these tags but they could cause weird rendering of elements that follow them. If you don't close <img>, the browser might think that any following elements are children rather than siblings and display them accordingly.
Using document.write('<div>some html</div>'); instead of $('#parentDiv').html('<div> some html </div>');
document.write is just bad in general. Some browsers don't like it (chromium) and in all browsers it freezes all other execution while it is writing to the page.
Manually constructing an html sequence rather than passing a string.
The former is not incorrect per se, but it is much, much slower than passing a string to .html(), at least according to the jQuery Cookbook.
$parent = $(document.createElement('div'))
.appendTo('#someId')
.attr({id: 'foo', class: 'foobar baz'})
.text('hello world');
According to jQuery Cookbook this is much, much faster
$parent = $('#someId').append($('
'));
FYI, prefixing cached jQuery reference w/ a "$" is a good practice.
Putting style information inline
This makes the lesson much harder to debug as you have to look both in the .css file and at the html. As the lesson grows, which it will, it gets harder and harder to pin down small bugs
Mixing naming conventions, like camelCase and not_camel_case
Neither of these naming conventions is the "correct" convention but mixing them makes it hard for others to work w/ your code