When someone bought my 2D game, their key file didn't work at all. I checked their key file against my known, working one and there was not one difference. The user was experiencing 14 differences causing it to not run in full edition mode. I even used the key file the one sent me and it ran just fine on my system. Why does it work on my system and not on another? This will cause a big problem when it comes to ordering it. I had to generate another key to fix this.

For example, you are registered in your windows as Administrator and may write to registry under HKLM. Your customer registered as User and haven't write permissions to the HKLM branch, so installer/updater can't write to.
There are many such 'reefs', depends on technique was used to make install/update.

Nothing gets written to the registry. The program just reads from a file and changes key strings and variables within the program itself (and only that program). I don't have an installer either as I don't have any nor have any real need for one, maybe just for creating shortcuts.

- are some files write protected?
- is the file- & directory-structure the same as on your system (maybe you try your game on a "clean" system)

mercuryus

ps: sometimes user devise errors to be able to give back the product...

No files are write-protected. The user who had this tried the game on another system and the original key worked without any problems on that computer. The main computer the user had didn't for some reason. There's otherwise no directory structure - the key must be in the same directory as the the game itself. It's just two files - the EXE itself and the key file. Other files do spill out as well from use such as game save files (including a single backup file), the error log (of which I used to resolve that user's issue), etc., all in the same directory as the EXE itself. This is the one main thing I don't get at all.