Re: nik's levels sets
A good rule of thumb for making any level is that you should be able to beat every single level twice before you post it. (Of course, when I make my hard set, I will completely scrap this rule because I have insufficient skills to beat really hard levels...) Beating a level a second time means that it was not some lucky one-off fluke that you will never be able to recreate again (e.g. a weird jump).
Totally agree. Testing your maps to see if their possible in any videogame is very important. You'll also be able to see any problems by testing the level too. Another good suggestion is that if the level is supposed to be possible to get all coins, test you level to if it is possible to get all coins.