Mailing labels - print: "TCPDF ERROR: Some data has already been output, can't send PDF file"
I imagine the ideal solution would be to sniff out the reason there's content in the output buffer when there shouldn't be. That's not what I did. Knowing very little about PHP, I looked up functions pertaining to the output buffer, then added 

ob_clean();


right after the author/version comments in tcpdf.php ... that's the only change I had to make. Successfully downloaded my address label pdf in Civi and patted myself on the back.