I can see one situation where things will not work as expected: When pushing the Sum button without having a range selected, expecting Calc to automatically select the correct range.
In this case, Calc will not read your mind (even though it seems like it does sometimes). Instead it will select a contiguous range of numbers from the active cell, upwards or to the left. So if there's no empty cells in the range, Calc will do what's needed. If there are empty cells, Calc will stop at the first empty space.
If a wrong range is selected, you can manually select the correct range before accepting the formula.
Apache OO 3.4.1/4.1.2, on Ms Windows 7/10 and Mac OS-X. LibreOffice 188.8.131.52 on Mepis Antix MX