Back at the start of January, a server bug gave about 50 commanders over 5 billion in credits. The money was taken back after a big backlash from the forum community.
Just because the client is 32 bit doesn't mean it can't handle 64 bit numbers.
The overall credit count is at least 64-bit, but there are(were?) 32-bit representations of certain things, e.g. module rebuy at one point, which resulting in "buying" things for -2Billion credits. I saw it once but just ignored it as I don't fudge around with exploits, but it was fixed pretty quickly. I suppose some people just bought/sold/bought/sold making 2^31 credits each time.