The errtest subdirectory contains a number of primary HDUs designed for checking the ability of FITS readers to cope with the unexpected. It includes one that is in compliance with the standard and a number of others that are identical except for deliberately inserted errors in syntax. One is simply a text extract, to test the behavior of the FITS reader when confronted with such a file, as might occur if such a file were mistakenly retrieved instead of a FITS file. An aaareadme.doc file describes the contents of this subdirectory. These files must be retrieved in binary format if they are to work properly.
Many Web browsers interpret FITS files as text by default.