Another glaring exception is Interstate 238 in California. It's an important, if short, route in the Bay Area, but at the time it was redesignated an Interstate, every x80 number had been taken, either by other local freeways, or by unrelated highways. So, when it was updated to interstate standards, they just decided to let it keep its old number, even though interstate 38 doesn't even exist.

Since then, Highway 480 (it was never formally adopted as an Interstate) has been removed, but renumbering would be more trouble than it's worth, so it's remained I-238.