[ JDK VERSION : <1.7.0_21 ]

Oftentimes, when building via "Clean and build" on a big projects (which
depends on another local project, as a library) I get an error stating that the
MyOwnLibraryWhatever.jar cannot be deleted, as if a filesystem lock was on it
or something.
Sample build output:

ant -f C:\\Users\\LT\\Documents\\NetBeansProjects\\MyProject clean jar
init:
deps-clean:
Updating property file:
C:\Users\LT\Documents\NetBeansProjects\MyProject\build\built-clean.properties
MyOwnLibraryWhatever.init:
MyOwnLibraryWhatever.deps-clean:
Updating property file:
C:\Users\LT\Documents\NetBeansProjects\MyProject\build\built-clean.properties
Deleting directory
C:\Users\LT\Documents\NetBeansProjects\MyOwnLibraryWhatever\build
C:\Users\LT\Documents\NetBeansProjects\MyProject\nbproject\build-impl.xml:1396:
The following error occurred while executing this line:
C:\Users\LT\Documents\NetBeansProjects\MyProject\nbproject\build-impl.xml:1432:
The following error occurred while executing this line:
C:\Users\LT\Documents\NetBeansProjects\MyProject\nbproject\build-impl.xml:1385:
Unable to delete file
C:\Users\LT\Documents\NetBeansProjects\MyOwnLibraryWhatever\dist\MyOwnLibraryWha
tever.jar
BUILD FAILED (total time: 0 seconds)

Insisting, i.e. keeping repeating the Clean and Build action again, for a few
(3 or 4) times, will eventually result in a successful build (or simply waiting
long enough for this "lock" to be released).