Ahh, a YaBB user. The error lies in YaBB NOT being coded for warnings or strict. But what's going on is you're trying to print your message BEFORE the headers are actually printed. And when you add the headers, you really are doubling them making the real headers push through as text.
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Look at the boardindex function for more information on where the headers are coming from :)
"Age is nothing more than an inaccurate number bestowed upon us at birth as just another means for others to judge and classify us"