If you have forgotten to stop the time registration when you started working on another activity, then the time registration will be inaccurate. The previous activity will have too much time registered on it, and the newly worked on activity will have too little. In this case you can split the time trace that you forgot to close off into two separate time traces.
Splitting time traces can be done via the "Split time trace" window, which can be accessed via both the "Time trace" window and the "Manage time traces" window. When splitting a time trace you can define at which time you want to split the time trace, and possibly also change the activity for each separate time trace.