Quite a big one in fact. Just because you know what something looks like well enough to recognize it, doesn't mean you know what it looks like well enough to draw it without looking at it. The only way you get to that point is by drawing it over and over, either from life, or, from a good reference photo.
Below are two sketches of the same puppy. The one on the right was done without the reference photo. The one on the left was done with the reference photo.
You can see me drawing both of these in the video below.
Painter of portraits and wildlife