Erik, you should quote me completely too. I said >256 ticks ocassionally, and that's no bug, notaben rock-solid.
For example, if timer overflows in 256 ticks and the isr is like

if previous isr was >256 ticks
   isr is only 30 ticks, 
else 
  isr can be anything up to 512-30 ticks 
(all minus maximum possible latency)

and timer interrupt is the only interrupt or only higher priority interrupt, it is perfectly correct and functional.

Of course, having more interrupts of the same priority (concurrent) is always tricky and needs extra care...

Jan Waclawek