I'm calling it a flaw because it's evidently intentional behaviour, but it's very peculiar.
My disc is about 75% full, with about 12% more reserved for timers and the rest free, according to the red/yellow/green space bar, and I am occasionally getting disc full messages when I try to book a recording.
Sometimes it allows a partial entry for say half the programme, setting a 'time' type entry and a modified end time, stating that the recording will be allowed to continue until the disc is full, and this it does despite the modified end time. It does mean though that AR can't be used in this instance. Other times it doesn't allow the entry at all, yet there is plainly at least 12% free for the programme to fit. You need to delete a timer before it will allow another to be set.
Some of my timers are daily and weekly repeats, and a quick calculation shows that these are being factored into the timer reservation count for the disc, accounting for the no room message, but are not included in the yellow part of the space bar.
The bottom line is that it won't allow any more timers to be set if it thinks there is no more room to record them all for the week ahead.