It is good practice to install modules using the -preview option prior to doing the install proper. This reports the modifications that will occur on the WAR without making any physical changes. The changes that are of most importance to note are those that are going to update existing files.
As a general rule it is considered bad practice to overwrite an existing file in the WAR, however it is sometimes necessary. The MMT makes a backup copy of the updated file and stores it in the WAR. When an update of the module occurs and the old files are removed, this backup will be restored prior to the installation of the new files. Problems can occur if multiple installed modules modify the same existing file. In these cases a manual restore might be necessary if recovery to an existing state is required.