According to the FHS:
Programs must not assume that any files or directories in /tmp are preserved between invocations of the program.
IEEE standard P1003.2 (POSIX, part 2) makes requirements that are similar to the above section.
Although data stored in /tmp may be deleted in a site-specific manner, it is recommended that files and directories located in /tmp be deleted whenever the system is booted.
FHS added this recommendation on the basis of historical precedent and common practice, but did not make it a requirement because system administration is not within the scope of this standard.
However, this is only a recommendation, and furthermore, it states only between invocations of a program -- which unless you're rebooting your server as well at 5am on Sunday, isn't happening. So, things like lockfiles, temp sockets, etc get deleted and cause problems.
As MadHatter suggested, use