decades-old bugs in code
Timsort invented in 2002... implementation bug found in 2015 (by formal methods)
See also 20 year old bug in Java's mergesort discovered in 2006: "Nearly all Binary Search and Merge Sort implementations are broken"
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko