By default Adium replaces two hyphens with an em-dash in text you type or paste. This leads to subtly confusing behaviour when pasting commands:
foo --bar
becomes “foo —bar”, which can look very similar to foo --bar if your contact is using a terminal that supports variable-width glyphs for exotic character points.
It’s a quick fix: stop double-hyphens being transformed to a long dash by right clicking the Adium input field, and unticking Substitutions > Smart Dashes from the menu.