There was this one particular line of code that in both sessions caught my eye and was emphasized by the reaction of my pairs: "This should have a code comments on it", "Ehh, what does this do, I have no idea!". It was a regular expression verifying if a message should be parsed to passed or failed but the selection of what the sought for keyword was was by no means obvious.
I mentioned this out loud a few days later, just to seek for confirmation that instead of the proposed code comment, it should really just be captured in a convenience method that would have a helpful name. But as we talked on the specific example, we also realized that it would make sense to add a unit test on that regular expression to explain the logic just a bit more.
The unit test would start failing if for any reason the messages we used to decide on pass/fail would no longer be available, and would be more granular way of identifying where the problem was than reading the logs of the system test.
A regular expression drive-by made me realize we should unit test our system tests more.