Imbalance-USD: Forced Split?

[Steven Sforza]

Hi guys,

Been using GnuCash for a couple of months now, so far been pretty happy,
but every now and then when adding entries it does something that I'm
just not understanding.
I'll enter something like I normally do, say for example a withdrawl of
$10 using my ATM card, and for some reason out of nowhere, for no reason
that I can figure out, it tells me this is a split, and that I have to
enter it as one. 
I don't even know what a split *IS*, so I tell it to take care of it
automatically, and it always ends up being the same entry anyway (a
debit of $10) but it's forced into being under the category of
"Imbalance-USD." 
What the heck is going on here? How do I make it stop?

Thanks,
-Steve