IBM System/3 using EBCDIC. The machine was working perfectly for 30 years in one factory, but they needed to change several offsets due to a change in their supplier of metal sheets which are now a tad bit shorter than the original program was written for, and the guy that used to do those adjustments died in the mean time...

Either way, it was a lot of fun... Everybody should try it, it really puts into perspective what programming is all about