Some plugins (i.e. numbered bookmarks and debugger) use the wrong icons when used together.
"Geany should set all unused markers to SC_MARK_AVAILABLE and check
availability before using markers so that it cooperates with plugins
using markers. Geany should not reserve markers it isn't using since
this limits the number available to plugins. Geany must handle marker
exhaustion" (Lex Trotman answer in editors.geany.general)