Don't Suppose Anything
I pretty much had complete autonomy when creating Noopsi's syntax as the only active programmer working on the code base, that was both a blessing and a curse. The biggest problem being I pre-supposed a lot of things I shouldn't have, after all, if I thought it would be useful, it must be useful to everyone, right?
Of course not, for example, I decided semi-early on that two spaces in a row should create a newline, after all, Noopsi relies on newlines for telling where entries and items start and yet, many times the user can't add a newline (Twitter and IM come to mind) but could do two spaces in a row quickly and easily. I had found the perfect solution to the problem because no one would purposely put two spaces in a row.
After a few minutes of using Noopsi, Dave (Noopsi co-founder and destroyer of web applications) started complaining about strange blocks of newlines in some of his items. When I went to investigate I found he had been pasting into Noopsi output from the command line that he wanted to save, each line of the output starting with about 15 blank spaces.
The biggest lesson here: don't suppose anything, you will never think of every way your users are going to use your new web application. While we could have kept the two spaces shortcut, we decided it would be considered unexpected behavior more often than something that would make the user go, "cool, what a great shortcut!"