Sustainable pace is one of the most volatile practices in XP. I say this because it is the first thing that project managers look at when dealing with scheduling concerns. It never fails that when a project seems to fall behind, there is very little support from the PMO to reduce the number of story points a certain iteration will contain.
Lets face it, 99% of the time one is unable to accurately predict how long a task will take to fulfill and it is unreasonable for anyone to actually think that they any estimates beget any sort of truth. Mitigating risk is a huge part of the PMOs daily job but XP does not say that increasing the number of hours worked during any given iteration will yield successful results. One in fact increases one's risk for a few reasons:
- Developers get burned out a produce crappy code
- People quit because they can't take the overtime
- Buggy code shows up as developers take short cuts to meet deadlines
- Test cases become hard to maintain