Go program demonstrating the issue

repro.go

package main

import (
	"bufio"
	"bytes"
	"crypto/sha1"
	"fmt"
	"io"
	"log"
	"os"
	"sort"

	"github.com/cznic/kv"
)

type key [sha1.Size]byte

func main() {
	if len(os.Args) < 2 {
		log.Fatal("usage: repro dbfile")
	}

	keys := mustLoadFortunes()

	db, err := kv.Create(os.Args[1], new(kv.Options))
	if err != nil {
		log.Fatal(err)
	}

	const (
		// n = 795 // ok
		n = 796 // fails
		// n = 797 // ok
	)

	expected := 0
	for i, k := range keys {
		if i == n {
			break
		}
		if k[0] >= 0x10 {
			expected++
		}
		if err := db.Set(k[:], []byte("doesn't matter")); err != nil {
			log.Fatal(err)
		}
	}

	k := key{0x10}

	e, _, err := db.Seek(k[:])
	if err != nil {
		log.Fatal(err)
	}

	got := 0
	for {
		_, _, err := e.Next()
		if err != nil {
			if err == io.EOF {
				break
			}
			log.Fatal(err)
		}
		got++
	}

	fmt.Printf("got: %d (expected: %d)\n", got, expected)

	if err := db.Close(); err != nil {
		log.Fatal(err)
	}

}

func mustLoadFortunes() []key {
	f, err := os.Open("/usr/local/plan9/lib/fortunes")
	if err != nil {
		panic("misfortune!")
	}
	defer f.Close()

	s := bufio.NewScanner(f)

	var ks []key
	for s.Scan() {
		k := sha1.Sum(s.Bytes())
		ks = append(ks, k)
	}

	if err := s.Err(); err != nil && err != io.EOF {
		panic(err)
	}

	sort.Sort(withBytesCompare(ks))

	return ks
}

type withBytesCompare []key

func (a withBytesCompare) Len() int           { return len(a) }
func (a withBytesCompare) Less(i, j int) bool { return bytes.Compare(a[i][:], a[j][:]) < 0 }
func (a withBytesCompare) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }