Random calculation error with adding and subtracting times

I have a staff timesheet which records overtime/undertime worked by a member of staff based on their daily hours.

As with all timesheets, it has to do calculations with negative times, and therefore it uses the 1904 date system rather than the 1900 date system. This works perfectly everywhere... well, NEARLY everywhere. There is one row which generates a wildly wrong answer. It adds –00:04 to +01:14 and comes up with the crazy answer of –22:50.

I have attached the timesheet, with personal information removed. I apologise for attaching such a large spreadsheet, but everything is so interlinked, there's no way to cut down the amount of data without changing the data and thus making the problem invisible. The error is on the August worksheet in cell AG11; I have highlighted it in yellow and made it nice and big so it's easy to see.

Can anyone help figure out what's going on here, and how to fix it? Thanks in advance!