Maybe it would be worth adding a wiki article for generic "common modding mistakes."
I can't think of any off the top of my head, but I certainly know that if I encountered something, and then saw a ton of people with the same problem, I'd take the extra few minutes to add it to a list of commonly occurring user errors.
Actually while I was writing this post I thought of one.
The classic:
(incorrect)
ModLoader.addRecipe(new ItemStack(genericItem, 1), new Object[]{ "#", '#', Block.dirt.blockID});
vs.
(correct)
ModLoader.addRecipe(new ItemStack(genericItem, 1), new Object[]{ "#", '#', Block.dirt});
Where Block.dirt represents an Object within an array of Objects, but using Block.dirt.blockID will return the integer value of the Block's ID within the block array. Using the wrong code will compile with no errors, but will crash the game when it preforms sanity checking on recipes during start-up.
Maybe I should go open a talk page on this in the wiki...