TIP #50: Opatch failed after uncompleted/interrupted patch

I faced with a problem today during my AS 10g upgrade which involves applying different patches sequentially.
Opatch failed with the following message :

Problems with the lock file
Lock file exists, details are:
Interim Patch is holding the lock from xxxxxxx,
probably due to previous unsuccessful operation

ERROR: OPatch failed during pre-reqs check

Opatch creates patch_locked in $ORACLE_HOME/.patch_storage during running of patch.If for some reason opatch run was interrupted or canceled , this file may not be removed.As the result, further opatch run can find this patch and will fail because of this file. Solution is to delete this file.