Quick answer
Relying on someone unreliable when it matters is not neutral, it is painful. The two images are both about a part of you that fails under load.
Read both lines
Both pictures are things you already have, working badly. A bad tooth is fine until you bite; a dislocated foot is fine until you stand. That is the saying's whole insight: the failure is not visible in advance and it arrives precisely when weight is applied.
The setting is בְּיוֹם צָרָה, in the day of adversity, the same phrase 24:10 used, which sharpens it further. This is not about a friend who disappoints you casually. It is about the day you needed someone, which is also the only day you find out. The verse assigns no blame to the person who trusted, and it does not tell you how to know in advance, because it cannot.
Related: Proverbs 24:10 · Proverbs 20:6 · Proverbs 17:17 · Proverbs 18:24
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.