The question is a little vague. Any of these can either display errors or cause the report to not run.
Use Control Name in detail section of report for totals field (example: Name is Overtime so total of overtime should appear like =Sum([OverTime]))
Be careful not to name your controls the same. Detail control can be named Overtime. Your total of overtime should NOT be named overtime as well. Name it like Tot_OverTime otherwise you could end up with a circular reference.
If counting a text field, know that null fields are an issue. Suggest counting identity seeds or fields you know always have a value.
Make sure each control in detail you plan to sum, is formatted for numbers. If you look at the control property, format should be a number and you can assign decimal places. If not, you need to check your query or source table to ensure field is properly assigned a format.
One good tip. Add one field to total at a time. A little tedious but ensures one works before adding the next. Always recommend this to newbies until they get real comfortable making reports. Easier to debug.
Good Luck.