Door knobs sometimes fall off, but they can easily be replaced. Learn how to fix a door knob that fell off.
How to Replace a Broken Door Latch, DIY Door Latch
The Swivel Spindle, or My Knob Just Came off in My Hand! - Dengarden
How To Fix or Replace A Mortise Doorknob
My friend's doorknob fell off the door in his 75+ year old house. How would I fix this? (more in comments) : r/DIY
How To Fix A door Handle That Fell Off
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How to fix a door knob: A secret weapon that will lock it in place!
Kwikset Door Handle Fell Off (Easy Fix)
The Swivel Spindle, or My Knob Just Came off in My Hand! - Dengarden