After updating a site to Drupal 7.99 and trying to generate a PDF, I get the following error:
TCPDF ERROR: Some data has already been output, can't send PDF file
A difference with Drupal 7.98 is that in modules/field/field.crud.inc all install files get included instead of only the field's module install file.
Before:
module_load_install($field['module']);
After:
module_load_all_includes('install');
I found out that when closeblock.install was not included during this process, the error went away.
I have not tested it on a clean install yet, but I would think it could be reproduced by generating a PDF using the TCPDF library and including closeblock.install before outputting the PDF.
Recreate the closeblock.install file to destroy any invisible characters.
Issue fork will follow.
Upon further investigation using Sublime and the RawLineEdit plugin, the character that appears before the PHP open tag is called "0xfeff", which appears to be a "Zero width no-break space". The code in my MR removes it.
See also the screenshot: