What makes a mod incompatible with KSP?

[Thomas988]

I'm just curious. What causes mods to not work properly most of the time when a new version of KSP is released?

Edited May 14, 2015 by Thomas988
Title changes

[michaelsteele3]

Because they might look for files that have been changed and/or removed.

[DMagic]

More likely is that they look for methods or fields in the code that don't exist or have different parameters. This causes an error whenever an addon tries to use those methods. Sometimes it's just a one-time error that doesn't really affect anything else. Other times it's something more serious, errors during the loading process can interrupt the entire loading sequence, breaking many things.

For instance, the latest update changed how the maximum atmospheric height was calculated. The old method was removed, so whenever something tried to access that method it failed.