People where I work have the annoying habit of naming files the same as the class they define, except that the filename is capitalized differently than the class. So the class ZooAnimal is in files zooAnimal.h and zooAnimal.cpp. Why? Why???
It's annoying enough when I'm trying to locate the file for a given class, but a more frequent annoyance is that when I hit M-/ to run dabbrev-expand, the out-of-the-box behavior is to change the case of what I typed to match the expansion that dabbrev found. Using the example above, if I type "ZooAn M-/", dabbrev notices #include "zooAnimal.h", and gives me the completion "zooAnimal". Grrr...
The dabbrev package is so fundamental that you don't need to specially enable it in your .emacs file. In fact, I didn't even realize that's what I was using, so in my first efforts to change the completion case-folding, I was searching for variables named "*complet*", and hit a dead end. Only on the next day did I think to C-h k M-/ to find out what function was case-folding the expansions, and that led me immediately to the variable dabbrev-case-fold-search. The default setting for that is the symbol 'case-fold-search', which means to use the same value as case-fold-search. Now, I do find case-folding useful on searches, either for laziness' sake, or because you don't always remember the exact capitalization of what you're looking for. So I have case-fold-search set to 't', meaning the default behavior of dabbrev-expand was very annoying.
Solution: either enter C-h v dabbrev-case-fold-search and click the customize link (and choose "off" for "case is significant" -- both of which are confusing descriptions in my opinion), or in your .emacs put "(setq dabbrev-case-fold-search nil)".
I purposely left dabbrev out of the post title, hoping people who are confused like me will be more likely to Google it.
