What terrifies you the most as an IT leader when an employee isn't properly deprovisioned systems access after offboarding?

Good question. They all worry me.

In addition to the security threat, the cost of unused licenses and unused resources like allocated storage, virtual machines, etc. is something that also needs to be considered with off-boarding. While the impact on a smaller organization may not be significant, for larger organization it does make an impact. 
Also the fact that while there is a small probability value associated with security breeches due to unused accounts, the probability of the resource leakage is 1.

Most terrifying is having to spend countless time explaining to auditors what has happened, how it happened, and what you are doing about it to make sure it doesn't happen again. That is by far the biggest pain that you come across most frequently you fail to off-board folks properly. All others happen sometimes but not nearly frequently enough as being grilled by some audit folks who are happy to pick up on this easy find and grill you on it.