Every winter batch, the same two items sink someone's trek: wet cotton socks and a headlamp with dying batteries. Neither is dramatic. Both are completely avoidable, and both are the actual reason people turn back — not altitude, not fitness.
Layering on a winter Himalayan trek is three systems, not one big jacket. A moisture-wicking base layer against skin, an insulating mid layer (fleece or a light down), and a windproof, water-resistant outer shell. The mistake we see constantly is one thick jacket doing all three jobs — you overheat on the climb, sweat into the insulation, then freeze the moment you stop at camp.
Feet matter more than anything else on the packing list. Two pairs of trekking socks (wool, never cotton) rotated daily, gaiters if there's fresh snow, and boots broken in weeks before the trek — not the week of. Blisters at 3,500m in sub-zero temperatures are a genuinely different problem than blisters on a weekend hike.
For Chadar specifically, add a sleeping bag rated well below the stated night temperature (caves and tents on the ice run colder than the number on the label suggests), a full change of thermals kept dry in a drybag for emergencies, and micro-spikes even where the crew provides them — having your own means one less thing to queue for.
The gear people over-pack: a second pair of trekking poles, guidebooks, a camp chair. The gear people under-pack: a power bank rated for cold (lithium batteries drain fast below freezing), lip balm with SPF, and a headlamp with fresh batteries plus one spare set carried on your body, not in the duffel that lags behind on the mule.