Undo

Ctrl+Z undoes your last change in the glyph you're editing. Each open glyph keeps its own history, so undoing in a never touches b.

Actions on the font as a whole (deleting, moving or renaming glyphs, changes in Font Info, a script run) are undone from the main window too. When an action can be undone, the notification that confirms it often has an Undo button.