I had a patch in presentation mode, one object was selected. In the Inspector I unchecked the box "Include in Presentation". Max crashed, the patch was damaged. As I tried to open it Max said:
Test Preset: bad magic number
Test Preset: read failed error 0
I opened the damaged file in BBEdit, code looked ok, pasted it into a new Max patch. Saved/replaced the old file. Did not went well, as I reopened it Max complained about a bad header. Deleted the old file, new patcher, pasted the code and saved as a new file. I am back in the game. Unfortunately I could not reproduce the crash yet.
What is a magic number?