I think we've all done that at some point in this dance.
Refrigerating opened Lantus is just to give us as long of a shelf life as possible since we use so little of it at a time with cats. Humans using Lantus may not refrigerate at all once they've "opened" their Lantus since they use it faster. As tiffmaxee said, note your spreadsheet so if you see a trend in the numbers later that look like your insulin isn't working anymore, you will see why it might have a bit shorter life. (I never noticed it shortening the life when I'd forget the vial out occasionally, so you are probably fine).
Lantus cannot be frozen though.