This happens infrequently, but enough that it is quite annoying:
I make a notebook and somewhere within it I have a line such as:
If I evaluate that line and then run f it outputs 27, as it should. If I save the notebook, sometimes when I reopen it the next day and evaluate the above expressions, the output for the second one is "f", not 27. I have to go back to where I set f[x_], erase either the colon or equal sign and reenter it, then evaluate it again for it to work. This gets quite tedious.
Has anyone else had this problem? If I copy the contents of the cell into a new cell and erase the original, it does not fix it. If I copy just the contents to the right of the := and rewrite the left portion and the :=, it does not fix it.