My favourite - and I've gotten into arguments with people about this who clearly never just tried it to see what happens - is it never executes things with the shell you think it will.

So many people assume that just because their script says #!/bin/bash at the top that cron is going to run it in bash. The reality is without ALSO setting SHELL=/bin/bash in the crontab file, you're getting your system's lowest-common-denominator shell (ash/dash/sh/whatever other gross abomination).

So much time wasted debugging. And I'm generally pretty good at avoiding shell-specific syntax, I've seen the abominations of shell scripts some people write.

The one thing I do wish systemd timers offered is cron syntax backwards compatibility, rather than just its ISO8601-style time patterns. An every-5-minutes job that used to be */5 * * * * is now OnCalendar=*-*-* *:0/5:0 and I'm just not sure that syntax is universally an improvement.