With the recent attacks by governments and corporations on our freedom, I feel like this is more relevant than ever.

"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." (Benjamin Franklin)

An overflow in C, the program was writing 16 Bytes in an array of length 10 due to some mistake.
The problem was this ended up overwriting another place in the memory that was used by another algorithm to perform calculations... So we thought that the algorithm was buggy all along.

Coworker: I did not progress much but at least I managed to get rid of all compiler warnings.
Me: That's okay. What were they about?
Coworker: No idea.
Me: How did you get rid of them then??
Coworker: I removed the "Wall" option when I use gcc.

Commit early and often

Using atoi(), atof(), atol() in production code. Everywhere.