Hi Ayla,
...when I am in Ruby on Rails scope, I cannot type lowercase 'a'! Hence, I cannot type :all, or 'class', or any word that contains a lowercase 'a'.
[Well, if no one else is offering...]
I can only suggest that somehow a command has been given a "key equivalent" of "a", which is somewhat bizarre (but I did have a similarly painful discovery when the latex2 bundle mapped "$" to "match previous latex ending bracket", meaning that I couldn't type a simple "$" sign at certain stages of writing a document. Even more frustrating, the cursor would jump back to the bracket match, so any subsequent typing appeared in the wrong place).
Anyway... try seeing if there is a command mapped to "a" by opening a document, choosing command-control-T, and choosing "key equivalent" from the magnifying glass. Then just type in "a". If you strike gold then open the bundle editor and change the key equivalent of that command.
Worth a shot...
Cheers, Paul