diff --git a/src/class/cc.c b/src/class/cc.c
index e0451697..efd78a0c 100644
--- a/src/class/cc.c
+++ b/src/class/cc.c
@@ -60,7 +60,7 @@ static void get_include_deps(compile_task_t* task, char* cc) {
 	// -MT "": Exclude the source file from the output.
 	// TODO Can we do this in parallel easily with 'cmd_exec_async'?
 
-	cmd_t __attribute__((cleanup(cmd_free))) cmd;
+	cmd_t __attribute__((cleanup(cmd_free))) cmd = {0};
 	cmd_create(&cmd, cc, "-fdiagnostics-color=always", "-MM", "-MT", "", task->src, NULL);
 	add_flags(&cmd, task);
 
diff --git a/src/frugal.c b/src/frugal.c
index 4a5bec74..05d5120a 100644
--- a/src/frugal.c
+++ b/src/frugal.c
@@ -59,6 +59,9 @@ bool frugal_flags(flamingo_val_t* flags, char* out) {
 
 	fclose(f);
 
+	// Note that we want to compare the order of these flags too, so just compare the strings.
+	// There could be some cases where flipping the order of flags could change the build, which is perhaps rare but let's not try to be too smart here.
+
 	if (strcmp(prev_flag_str, flag_str) != 0) {
 		goto write_out;
 	}
diff --git a/tests/frugality.sh b/tests/frugality.sh
new file mode 100644
index 00000000..3a86a80f
--- /dev/null
+++ b/tests/frugality.sh
@@ -0,0 +1,174 @@
+#!/bin/sh
+
+. tests/common.sh
+
+# Regression test for the following PR:
+# https://github.com/inobulles/bob/pull/70
+
+# Make sure build.fl's Cc flags are correct.
+
+if [ $(uname) = Linux ]; then # XXX Linux is annoying.
+	sed -i 's/-std=c11/-std=c99/g' tests/frugality/build.fl
+else
+	sed -i '' 's/-std=c11/-std=c99/g' tests/frugality/build.fl
+fi
+
+# Clean up.
+
+BOB_PATH=tests/frugality/.bob
+rm -rf $BOB_PATH
+
+# Functions for getting the mtimes of all cookies and preinstalled files.
+
+build() {
+	out=$(bob -C tests/frugality run 2>&1)
+
+	if [ $? != 0 ]; then
+		echo "Build failed: $out" >&2
+		exit 1
+	fi
+}
+
+get_mtimes() {
+	export src1_${1}mtime=$(date -r $BOB_PATH/bob/src1.c.cookie.6531c664ecf.o +%s)
+	export src2_${1}mtime=$(date -r $BOB_PATH/bob/src2.c.cookie.6531c665310.o +%s)
+	export linked_${1}mtime=$(date -r $BOB_PATH/bob/linker.link.cookie.33faaddbf5af8a68.l +%s)
+
+	export obj1_${1}mtime=$(date -r $BOB_PATH/prefix/obj1.o +%s)
+	export obj2_${1}mtime=$(date -r $BOB_PATH/prefix/obj2.o +%s)
+	export bin_${1}mtime=$(date -r $BOB_PATH/prefix/bin +%s)
+}
+
+update() {
+	# We're running here instead of just building (even if there's nothing to run).
+	# This is because we want the preinstallation step to run.
+
+	build
+	get_mtimes ""
+
+	# If the mtime of the installed file is less than that of the cookie, then it was not reinstalled, which is bad!.
+
+	if [ $obj1_mtime -lt $src1_mtime ]; then
+		echo "obj1.o's cookie was updated, but it was not preinstalled." >&2
+		exit 1
+	fi
+
+	if [ $obj2_mtime -lt $src2_mtime ]; then
+		echo "obj2.o's cookie was updated, but it was not preinstalled." >&2
+		exit 1
+	fi
+
+	if [ $bin_mtime -lt $linked_mtime ]; then
+		echo "bin's cookie was updated, but it was not preinstalled." >&2
+		exit 1
+	fi
+
+	# If the cookie was not updated, then it shouldn't have been preinstalled.
+
+	export src1_updated=
+	export src2_updated=
+	export linked_updated=
+
+	if [ $src1_mtime = $src1_prev_mtime ]; then
+		if [ $obj1_mtime -gt $obj1_prev_mtime ]; then
+			echo "obj1.o's cookie was not updated, but it was preinstalled." >&2
+			exit 1
+		fi
+	elif [ $src1_mtime -gt $src1_prev_mtime ]; then
+		src1_updated=true
+	else
+		echo "This is impossible." >&2
+		exit 1
+	fi
+
+	if [ $src2_mtime = $src2_prev_mtime ]; then
+		if [ $obj2_mtime -gt $obj2_prev_mtime ]; then
+			echo "obj2.o's cookie was not updated, but it was preinstalled." >&2
+			exit 1
+		fi
+	elif [ $src2_mtime -gt $src2_prev_mtime ]; then
+		src2_updated=true
+	else
+		echo "This is impossible." >&2
+		exit 1
+	fi
+
+	if [ $linked_mtime = $linked_prev_mtime ]; then
+		if [ $bin_mtime -gt $bin_prev_mtime ]; then
+			echo "bin's cookie was not updated, but it was preinstalled." >&2
+			exit 1
+		fi
+	elif [ $linked_mtime -gt $linked_prev_mtime ]; then
+		linked_updated=true
+	else
+		echo "This is impossible." >&2
+		exit 1
+	fi
+}
+
+build # Run this once to populate .bob already.
+
+echo "Test 1: changing 'src2.c'."
+
+get_mtimes "prev_"
+sleep 1
+touch tests/frugality/src2.c
+update
+
+if [ "$src1_updated" = true ] || [ -z $src2_updated ] || [ -z $linked_updated ]; then
+	echo "Changing 'src2.c' failed." >&2
+	exit 1
+fi
+
+echo "Test 2: changing both 'src1.c' and 'src2.c'."
+
+get_mtimes "prev_"
+sleep 1
+touch tests/frugality/src1.c tests/frugality/src2.c
+update
+
+if [ -z $src1_updated ] || [ -z $src2_updated ] || [ -z $linked_updated ]; then
+	echo "Changing 'src1.c' and 'src2.c' failed." >&2
+	exit 1
+fi
+
+echo "Test 3: changing nothing."
+
+get_mtimes "prev_"
+sleep 1
+update
+
+if [ $src1_updated ] || [ $src2_updated ] || [ $linked_updated ]; then
+	echo "Changing nothing failed." >&2
+	exit 1
+fi
+
+echo "Test 4: changing 'src2.h'."
+
+get_mtimes "prev_"
+sleep 1
+touch tests/frugality/src2.h
+update
+
+if [ $src1_updated ] || [ -z $src2_updated ] || [ -z $linked_updated ]; then
+	echo "Changing 'src2.h' failed." >&2
+	exit 1
+fi
+
+echo "Test 5: changing compilation flags for all source files."
+
+get_mtimes "prev_"
+sleep 1
+
+if [ $(uname) = Linux ]; then # XXX Linux is annoying.
+	sed -i 's/-std=c99/-std=c11/g' tests/frugality/build.fl
+else
+	sed -i '' 's/-std=c99/-std=c11/g' tests/frugality/build.fl
+fi
+
+update
+
+if [ -z $src1_updated ] || [ -z $src2_updated ] || [ -z $linked_updated ]; then
+	echo "Changing compilation flags failed." >&2
+	exit 1
+fi
diff --git a/tests/frugality/build.fl b/tests/frugality/build.fl
new file mode 100644
index 00000000..82fd8c26
--- /dev/null
+++ b/tests/frugality/build.fl
@@ -0,0 +1,13 @@
+import bob
+
+let src1 = "src1.c"
+let src2 = "src2.c"
+
+let obj = Cc(["-std=c11"]).compile([src1, src2])
+let linked = Linker([]).link(obj)
+
+install = {
+	obj[0]: "obj1.o",
+	obj[1]: "obj2.o",
+	linked: "bin",
+}
diff --git a/tests/frugality/src1.c b/tests/frugality/src1.c
new file mode 100644
index 00000000..f15123c5
--- /dev/null
+++ b/tests/frugality/src1.c
@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+void fn(void) {
+	printf("B\n");
+}
diff --git a/tests/frugality/src2.c b/tests/frugality/src2.c
new file mode 100644
index 00000000..440a21ab
--- /dev/null
+++ b/tests/frugality/src2.c
@@ -0,0 +1,10 @@
+#include "src2.h"
+#include <stdio.h>
+
+void fn(void);
+
+int main(void) {
+	printf(STR "\n");
+	fn();
+	return 0;
+}
diff --git a/tests/frugality/src2.h b/tests/frugality/src2.h
new file mode 100644
index 00000000..e3703f37
--- /dev/null
+++ b/tests/frugality/src2.h
@@ -0,0 +1 @@
+#define STR "A"