Well they had to taxi back on stand recently at Newcastle, having already set off for the runway and prepared for take off. Seems like the muppets miscalculated their fuel requirements and only noticed their error as they prepared to line up. Nice little delay for everyone whilst they taxied back to stand and waited for the fuel truck to arrive. Again.
I'm sure this could happen to anyone - after all, we're only human - however, that errors like this continually dog the Icelandic arm of Excel is no surprise and is indicative of the standard of operation that they are happy to accept as the norm.