The worst, as a piece of code, was a Byzantine C++ application. It had been written with abstractions and indirections to cater for every conceivable eventuality. And it seemed to incubate Heisenbugs.

Usually, with a C++ app you can fix it incrementally but we tried and it was just impossible. Far too intertwined. In the end I wrote a replacement from scratch.

The other one was a Smalltalk system. Actually, pretty well written and documented. But once Smalltalk gets above a certain size it can become impossible to reason about. And it’s very hard to replace parts. Sadly, this one was a failure. It never got fixed or replaced. Just went out of business.