Many a time and oft,
crontab scripts are not executed as expected. There are numerous reasons for that, for example: wrong crontab notation, permissions, environment variables and many more.
This community wiki aims to aggregate the top reasons for
crontab scripts not being executed as expected. Write each reason in a separate answer.
Please include one reason per answer - details about why it's not executed - and fix(es) for that one reason.
Please write only cron-specific issues, e.g. commands that execute as expected from the shell but execute erroneously by cron.