Grepping the source of the pages in man-pages will show various places where pages are marked with the string "FIXME". There is also a shell script (scripts/FIXME_list.sh) that can be used to obtain a list of FIXMEs in the man page sources:
The presence of a FIXME usually indicates that someone has noticed that some information on the page is incorrect or incomplete, but has not had the time/knowledge to fix the problem. (Sometimes a FIXME relates to a kernel or glibc bug report that is awaiting resolution, and it may be sufficient to check if the bug has been resolved and then, if necessary, provide a suitable write-up on the page.)
If you know how to fix the problem, then please send a patch. However, note that some of the FIXME markings are associated with problems that are quite difficult: you need to ensure that you are knowledgeable on the relevant point(s), or you need to be willing to invest the time to become knowledgeable (by reading kernel or [g]libc source files and/or writing suitable test programs).