When I first started learning kanji, everything in my book appeared so intricate and impossible to remember. But now, something as basic as 日, 女, or 水 seems impossible to forget. I don't know... If I happen to stay away from Chinese characters for twenty years, who knows, I might forget them. The sad thing is, there are much more complex things than those radical characters.
Picture this situation: You're in a classroom, doing a proficiency test on written Japanese, and you can just hazily remember the outlines of the kanji character you're supposed to write on the test BUT can't seem to get it right no matter how much effort you put into remembering it. On the answer sheet, you've constructed a Chinese character that looks like the one you're looking for, but is a totally different character—or not even a recognized Chinese character for that matter.
Very often it's the radical that's causing trouble. (A radical (部首) is the principal part of a Chinese character which commonly denotes its theme, relation, element, or however you want to name it. For example, the radical of 焼 (to fry) is 火 (fire) because frying at least historically relates to fire. With characters that have obvious themes, such as the one I used in the bracketed example, it's easy to surmise which radical goes with the kanji. Sometimes, however, radicals are totally irrelevant. The radical for 好 ('to like' in Japanese and 'good' in Chinese) is 女 (woman). Well, 好 is a combination of 'woman' and 'child' but some women have been convicted of leaving their sons to die alone in their apartments, and some kids in the recent past have gone totally wild with their knives and axes.
Fine... that might've been a bad example. Of course moms and children like each other natually, and they are a good combination most of the time. But I can't seem to find any clear connection between something like 験 (test) and 馬 (horse).
I personally find those characters with no intelligible connection between their radicals and the rest of their structures easiest to forget. How about you?