The packing list topic is so well-covered online that I want to focus on the 10% of items people consistently under- or over-value.

Most undervalued items: a small microfiber towel (hostels often charge for towel rental, and having your own means not waiting for hostel towels to be available), a universal power adapter with USB ports (one device that covers every outlet type globally), a packable rain jacket (not an umbrella — both hands free matters when carrying a backpack), and a small first aid kit with more than just bandaids (blister treatment, oral rehydration salts, antihistamine, ibuprofen, and Imodium are the five items that earn their weight repeatedly).

Most overvalued items: a dedicated travel towel that's expensive when a generic microfiber from a sports store works identically, security pouches worn around the neck (uncomfortable and conspicuous), multiple pairs of shoes (one pair of good trail runners handles 95% of travel situations), any guidebook (phone plus Wikivoyage), and travel-specific clothing lines that cost 3x more than equivalent gear from general athletic wear brands.

The packing mistake I see most often: packing for every possible scenario rather than for the most likely scenarios. If you're spending 90% of your trip in warm Southeast Asia and 10% in an air-conditioned environment, pack for Southeast Asia and buy a cheap sweater if needed.