Item cannot be saved because it has been modified externally: node
The ProblemI came across this nasty little error when altering some configs in the JCR content explorer and updating packages.
I had updated a setting in CRX/Explorer, not clicked the Save All button, and in another window installed and deployed a package. (That I didn't think altered any of these configs).
It turns out that it did, and whenever I tried to save my changes, I got the above error.
Right click on the offending node that's causing the issue, click on revert, and apply your changes.