Or, in my case, if I've given my mod a clean bill of health, run it through 3 beta testers and a virus scanner just to be sure. Which is why I now have a dedicated beta tester down the strret to give me face to face information on my latest crippling error.
You are your best level/beta tester: you know all the potential trouble spots and secrets that others may miss.
A varied set of playing styles is more effective for testing levels than your own. The other beta testers are more important than you are for the purposes of testing the mod. If you're any good at modding, you'll have played your own mod through several times during the course of making it. Also, you are not an effective judge of whether or not your levels are fun and playable (as I found with my "perfect timing" game, Stickman's Escape, a few years ago)
When the creator beta tests, he can check to make sure that everything works and looks how it its supposed to. Other betas testers can't do that. They instead make sure its fun and enjoyable, and possible for the average human being, possibly with a little legwork involved. Or a lot, if you, like me, find final levels/bosses annoyingly simple.
However, other beta testers also test wether the level is easy to do, something the creator thinks may be obvious, flick 3 switches and avoid the middle pit, may be frustratingly difficult for the average player. A lot of my early levels were like that.
Sometimes i wonder if you could beat your own level. That very irritating tower will all those shadow beings that drop down from above on you in the first castle of sk1 for instance, yuck.
i can get about halfway, or three quarters, basically aggravating them, then falling down like a million were afta me, shootin them, then jumpin up again. Then they drop down on you without warning. Its fun.
Thats the point when i think a beta tester would start asking for money