My top gotcha: If you forget to add a newline at the end of the crontab file. In other words, the crontab file should end with an empty line.
Below is the relevant section in the man pages for this issue (man crontab then skip to 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)