One of the first things you will notice about vi is that it likes to beep a lot and it tends to switch modes unexpectedly. vi will produce an error beep every time you do something that is not an option, such as paging past the end of the text buffer. There is nothing particularly wrong with that, except that vi often does more than just beep. As an example, if you try to move the cursor past the invisible end-of-line characters using the arrow keys, vi will beep and then switch to command mode. This wouldn't be that much of a problem if the mode indicator in the lower right corner always worked correctly, but it doesn't. Sometimes it works impeccably, but often it will say that it is in one of the text entry modes when it is actually in command mode. The danger here is that command mode, as mentioned before, interprets all keystrokes as commands. So if you are unexpectedly thrown into command mode and you keep typing, a series of powerful editing commands may destroy the document you are working on. This possibility reinforces the importance of backing up your documents frequently, so that you have the option of quitting without saving the buffer to disk. If vi beeps at you when you are in a text-entry mode, assume that you've been put into command mode, and reselect the text-entry mode by pressing the appropriate character. At worst, that character will be inserted into your document, where it can be deleted later. The consequences of one keystroke while in command mode can be far more destructive and time-consuming to undo.
Another quirk you may notice it that the up and down arrow keys do not work in text-entry mode; rather, they insert a line above where you are typing and put a capital letter on that line. You will need to exit text-entry mode, and delete the lines (with 'dd') to get rid of them, but otherwise you document will be unharmed.