When a user writes workaround code to deal with a software bug, and then the programmer later fixes the bug which breaks the workaround code. It is not hackward compatible.
User: This update broke the software!
Programmer: No, the update fixes the software, it simply is not hackward compatible.
From the game Portal 2. A lemon with the power to explode and start a fire. Yelled by Cave Johnson via an old recording while Chell and Glados navigate through the abandoned part of Apeture Science laboratories.
"I'm gonna get my engineers to invent a combustable lemon that burns your house down!"-Cave Johnson