Hayyim Vital tells us that his teacher was careful to wear his tallit katan (a four-cornered vest) over his shirt but under his other garments. In his words:
הטלית קטן היה לובשו למטה משאר מלבושיו ע"ג חלוקו שלא כאותם המתייהרים ללבשו ע"כ בגדיו וטעות גדול הוא בידם הפך האמת.
Vital accuses those who wear their tallit katan on top of their other clothes of arrogance. One wonders why it is arrogant to wear a tallit katan on top of your clothes but not arrogant to wear a tallit gadol (a sort of cape) on top of your clothes.