Tim Peters writes:

gvanrossum · gvanrossum · commit 2174dcb06187 · 1999-09-15T22:48:09.000Z
For a long time I've seen absurd tracebacks under -O (e.g., negative
line numbers), but very rarely.  Since I was looking at tracebacks
anyway, thought I'd track it down.  Turns out to be Guido's only
predictable blind spot &lt;wink -- "char" is signed on some non-GvR
systems&gt;.  Patch follows.
diff --git a/Python/compile.c b/Python/compile.c
@@ -3451,7 +3451,7 @@ PyCode_Addr2Line(co, addrq)
 	int addrq;
 {
 	int size = PyString_Size(co->co_lnotab) / 2;
-	char *p = PyString_AsString(co->co_lnotab);
+	unsigned char *p = (unsigned char*)PyString_AsString(co->co_lnotab);
 	int line = co->co_firstlineno;
 	int addr = 0;
 	while (--size >= 0) {

Original file line number	Diff line number	Diff line change
`@@ -3451,7 +3451,7 @@ PyCode_Addr2Line(co, addrq)`
`3451`	`3451`	`int addrq;`
`3452`	`3452`	`{`
`3453`	`3453`	`int size = PyString_Size(co->co_lnotab) / 2;`
`3454`		`- char *p = PyString_AsString(co->co_lnotab);`
	`3454`	`+ unsigned char p = (unsigned char)PyString_AsString(co->co_lnotab);`
`3455`	`3455`	`int line = co->co_firstlineno;`
`3456`	`3456`	`int addr = 0;`
`3457`	`3457`	`while (--size >= 0) {`