Not sure it's an error - just math + bonuses at the right time. I was lucky enough to see the tip about charging the XP building before getting the Prison Bus... bought a 3 day charge for 10D, then the bus got me 4 Bonus Box chances. So 10D got me 12D right away. Then I used several million I had saved up and went Bloodmobile crazy.
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Also see other reports @ http://forum.ea.com/eaforum/posts/list/10376798.page
And I still have 2 days of 5x XP!!!
I spent 5d for 24 hours and my donut count went up from 86 to 134 with the bloodmobile buy and sells.