Edited by DonRaphael - 5/31/15 at 10:37pm
If you narrow the forearms on the front seam, the pitch sleeve is slightly reduced. My diagram shows the effect somewhat exaggerated, but I guess you can see what happens.
It may or may not cause a problem, depending on the amount you want to slim and the original sleeve pitch. I had one jacket altered and then found this out the hard way
Which bit is the sleeve?