I constantly split my terminal windows accidentally by pressing Cmd+D (it's so close to Cmd+F). I have no idea how to undo this. Any ideas?
To undo the split, press Shift+Cmd+D (you can also do it from the View > Close Split Pane menu option).
If you wish to reassign the keyboard shortcut for View > Split Pane (Cmd+D) to something else to prevent anything from happening when you accidentally press Cmd+D, you can do it by going to System Preferences > Keyboard > Keyboard Shortcuts, adding the Terminal.app and entering the menu title (Split Pane) with a different shortcut.
Terminal keyboard shortcuts (from dashkards):