I noticed that if you enter "/roll d20", the d20 bit is interpreted as a static number instead of a die roll.
So if I do: /roll d20+5
The result always comes out: 20+5 = 25
Looks like the roll parser requires a leading number for dice. It should probably be altered to recognized die rolls with the leading number missing and handle them properly. It's easy to forget to type the leading 1, and easy to fail to notice when someone else has done this (though you'd probably get suspicious quickly).