Okay. Here is a scenario. Deviating 50 miles left of track for weather, using the offset function, ALT CRZ / NAV. Clear of weather. Relief captain decides to return to course, great. Does a "direct to" the next fix, which is about 150 miles down line. Airplane turns, active waypoint is that fix, "OFFSET DELETED" pops up in the scratchpad and all is well in the world.
5 minutes later I look down and notice that all my FMS flight plan predictions are dashed as if it is trying to calculate. His side is normal. "Hm". Minute or so later mine comes back, but is wildly different from his. Time estimates are now about 20 minutes off at each fix, EFOB at destination (YSSY) now showing -15,000 (minus) whereas his side now shows some ridiculously high number.
Break out a plotting chart, plot GPS position, all good and the NAV function is still working fine. Check flight plan numbers over previous waypoint vs ground speed, distance to the next fix, and fuel burn, numbers look fine. Still carry an E6B for superstition if no other reason, and found it handy at that particular time.
Tried everything we could think of to jog the system out of whatever brain-lock it was having (reset cost index, secondary flight plan, refresh direct, everything we could find in the QRH, Volume 3 or 4 etc). Nada, no change. "Hm."
Passed the waypoint. Waypoint sequenced but same discrepancies persisted. 20 minutes later, they crept back together, minute by minute. Soon, they matched up. Fuel calculations went haywire and then came back together, and all matched our manual running calculation. All was once again well in the world.
During this entire sequence there were NO ECAMS, no alerts of any kind, nada. No systems faults shown on the maintenance menus, no status page items, no scratchpad messages, nothing, zip.
Tell me how in the hell that was something we caused by "lack of knowledge" and I'll buy you a beer. Said "relief" captain was a TRE by the way, and I'm no slouch myself.
BTW the maintenance engineers had no clue, couldn't find anything wrong. Just pulled the boxes out and re-racked them for lack of any better solution. Was fine on the way home.