Running the game with commandline option --debug or -d enables debug mode within the game.
Debug mode displays additional information. It also enables context menu options to create units anywhere on the map, changing unit sides, and some additional CommandMode commands. If you create units in debug mode, you can then use the :unit command to set the unit's attributes (see CommandMode).
Debug mode can also be enabled within the game using :debug (see CommandMode), and on Mac versions of the game, by holding down the Option key when starting Wesnoth.
Note: Debug mode is not safe with replays. If you enter debug mode, create a unit, move it, and then save replay, the replay will be corrupt since the unit creation is not recorded and thus all actions related to it will cause OOS errors.
Debug mode is disabled when you play a network multiplayer game.
Building the game with extra debugging
The game can be built to support debugging by supplying --enable-debug as an argument to configure during the build process. Currently this enables a few code snippets for the benefit of an external debugging tool like gdb, and calls the compiler with -g and -O0 flags. You probably don't want to strip a debugging build, since the symbols are needed for the debugging tool.
An unstripped debug build on linux is over 80MB, compared to 2-3MB stripped. It is a good idea to run a debug build of the game if you do development, since it can be necessary to diagnose serious problems.