US Interstate highway 238 in California is an exception to the usual numbering rules. There is no interstate 38 anywhere, so I-238 seems to be a gross violation of the usual rule that three-digit interstate highways take their last two digits from the number of the main interstate they branch off from.

In this case, I-238 got its number because California wanted to have a small stretch of its state highway 238 designated as an interstate to close a gap in the interstate system near I-80, but all the x80 numbers were already in use in California, so they had to use a different number. Since 238 didn't conflict with anything, they used the number of the existing state highway.