Pitfalls

Don't	If	Explanation
llMessageLinked with a non-0 int	Ever	You can use llMessageLinked(x, 0, y, z), but negative and positive channels are usually reserved for runMethod. Use an input definition in the class header file instead, and call that method with runMethod instead.
Forget to define a PC_SALT and TOKEN_SALT	When using listeners	Otherwise your code might be vulnerable to injections
Create a global var named _TIMERS	When using multiTimer	_TIMERS is a reserved global list
Forget to wrap list, vector, rotation definitions in parentheses	When calling macros	The preprocessor will throw a fit. Use (<x,y,z>) (["X","Y"])
Forget to end a .lsl file without a newline	Ever	You'll get preprocessor errors
Stack heap	Writing bad code	See LSL Memory Best Practices