cron job occasionally not running

The original cron required each entry to end with a newline so yes sometimes you do need a blank line or something at the end.

   Although cron requires that each entry in a crontab end  in  a  newline
   character,  neither the crontab command nor the cron daemon will detect
   this error. Instead, the crontab will appear to load normally. However,
   the  command  will  never  run.  The best choice is to ensure that your
   crontab has a blank line at the end.

   4th Berkeley Distribution      29 December 1993               CRONTAB(1)

Some versions have it fixed or emit a warning for example Ubuntu Maverik (10.10) : crontab look at the diagnostics section at the bottom which states a warning will be written to syslog.

DIAGNOSTICS
       cron requires that each entry in a crontab end in a newline  character.
       If  the last entry in a crontab is missing a newline (ie, terminated by
       EOF), cron will consider the crontab (at  least  partially)  broken.  A
       warning will be written to syslog. 

This is the first answer that comes up with the search text cron error getpwname failed so I thought I would post the cause of my issue:

I was using /etc/crontab but had forgotten to put the user in front of the command.

i.e.,

*/5   *  *  *  * /bin/bash <filename>

Instead of

 */5   *  *  *  * root /bin/bash <filename>

It gave the same error, go figure.