Mistake: Zigzagging edge detail
When adding detail like little waves or jagged edges to a leaf, don't do so by applying a single continuous zig-zagging stroke. If you remember back to markmaking in lesson 1, the third rule is to ensure that each stroke consists only of one trajectory.
This is easy to separate when you've got sharp corners, but when you're dealing with more rounded waves as shown here, it may be a little less clear. In general, rise up off the edge from your previous phase of construction and come back down to it, then lift your pen and start a new stroke for the next bump.
Additionally, wherever possible, work additively - don't cut back into what you've already drawn, as this often makes us think more about the flat shapes on the page, rather than the solid, 3D forms they represent. In this case you'll notice very clearly that along the top edge of the correct example, I've come up off the edge and back down to it.
The other side however does seem to cut back into the leaf, but if you think about it in three dimensions, it doesn't. Instead, those edges are being lifted up slightly from their previous position, rather than being cut into.
There will be times when you cannot avoid having to work subtractively, but you should always do your best to see if there is a way to make additive construction work.