If you use a Unicode encoding in the Terminal you might encounter problems in IDL when sending commands after correcting wrong input that had contained a 'umlaut' (äöü) character or other non-English characters.
E.g. writing
ürint, 'He
...correcting it to...
print, 'Hello World!'
...and then sending it off, you will get
% Attempt to call undefined procedure/function: 'CPRINT'.
That is (I think) because Unicode uses two-byte characters and when deleting a two-byte character like an 'ü' IDL only deletes one of these two and the other one remains. This does not occur when you select a one-byte encoding like "Western European (Mac OS Latin)".