Do not touch, move, kick, cover, or pick up any munition or suspicious object. That is the rule. Everything else here is about the situations that make people break it.
Failed does not mean safe. A munition that did not explode on impact is damaged and unpredictable, and disturbing it is exactly the input its fuse was waiting for. Age is not safety either: ordnance from wars a lifetime ago still kills farmers every year.
Sub-munitions are the ones that get children. Cluster munition remnants are small, often brightly coloured, and look like a ball, a tin or a toy. Tell children, in plain words, that anything they do not recognise on the ground is not to be touched, and that they should tell an adult where it is.
In a suspected mined or contaminated area. Stop. Do not walk on. Retrace your own footsteps exactly, one at a time, back the way you came. Stay on hard surfaced roads that are in regular use. Take marking seriously — painted stones, tape, a sign, a stick in the ground — and treat any unmarked disturbed ground near a former position as suspect.
Damaged buildings after a strike. Do not enter rubble to recover possessions. Ordnance ends up inside collapsed structures, and booby traps are placed in exactly the objects people reach for.
Mark from a distance, and report. Note the location as precisely as you can from where you are safe, keep others away, and call the police or the emergency number. Describe what you saw — size, shape, colour, fins, wires — without going back for a better look.
Keep transmitting devices away from a suspected device and make the call from a distance. Some fuses respond to radio energy.
Do not photograph it up close, and do not post the picture with the location. Curiosity brings a crowd, and the crowd is the casualty list.
Source. United Nations Mine Action Service, https://www.unmas.org/ ; the Geneva International Centre for Humanitarian Demining, https://www.gichd.org/ ; the International Mine Action Standards, https://www.mineactionstandards.org/
What this is not. Reading, not training. Clearance is done by qualified teams under the International Mine Action Standards, and nothing on this page qualifies anybody to approach a device.