The originally attempt was with the Apprentice Clan Chymist.
Just to test it, I've just tried it with a gang having 75 credits and trying to buy a Apprentice Clan Chymist, Brute Handler and the Cawdor Stig-Shambler. All with the 'Purchase with Credits? ' option set to yes.
The Stig-Shambler wouldn't buy because I didn't have enough credits (as it should), while the other two both went through fine, but didn't deduct any credits. I have done this test on both Firefox and Chrome and gotten the same result.
The gang credits can just be edited to what they should be, so it's not a critical issue, but people might miss that it isn't deducted.