Credit Limit Weirdness & what is NaN?

[Luke <..hidden..>]

Is this a bug, a feature, or something unique to my installation?

Both in 1.2.12 and 1.2.13, I routinely see things like this, when creating 
orders or invoices:

Credit Limit 0 Remaining NaN

What is "NaN"?

Also, especially while working on orders, I see some very weird 
"remaining" values--starting at zero, I might add a product worth a 
hundred bucks, another worth fifty, another worth twenty five and change, 
update some quantities, and the like.  Then I look up at the "remaining" 
line, and see things like "9".  I do an update and then see "12", and on 
and on it goes--incrementing or changing in strange and nonsensical ways, 
even if I just do update after update, without actually changing anything.

However, the totals are right in all of these cases.

What might be going on?

Regards,

Luke