If you are a Mac Lightroom user running Blow Up from the Library module, you may encounter a Lightroom warning indicator that appears after Blow Up finishes resizing the image. The red arrow in the following picture points to this warning indicator. This warning indicator should not be displayed and this appears to be a bug in Lightroom. We have reported it to the Lightroom team.
To remove the warning indicator, click on it. This will bring up a message telling you that the metadata has changed. Choose the "Import Settings from Disk" option pointed to by the green arrow in the picture and the warning indicator will disappear. You should then be able to proceed with your work in Lightroom.