Hash Sum mismatch

[Martin <adsrtg-mlist@yahoo.com.au>]

When I tryed to install simutrans game with aptitude I got
this error and was not able to install it:

┌──────────────────────────────────────────────────────────────────────────┐
│E: Failed to fetch cdrom:[Lenny DVD                                       │
│   3]/pool/main/s/simutrans/simutrans-data_100.0+ds1-4_all.deb: Hash Sum  │
│   mismatch                                                               │
│                                    [ Ok ]                                │
└──────────────────────────────────────────────────────────────────────────┘

A few days ago same thing happend when installing hgsvn:

┌──────────────────────────────────────────────────────────────────────────┐
│E: Failed to fetch cdrom:[Lenny DVD                                       │
│   3]/pool/main/h/hgsvn/hgsvn_0.1.6-2_all.deb: Hash Sum mismatch          │
│                                    [ Ok ]                                │
└──────────────────────────────────────────────────────────────────────────┘

I have original DVDs. Content of /cdrom/.disk/info is:
Debian GNU/Linux 5.0.4 "Lenny" - Official i386 DVD Binary-3 20100131-19:16

Any idea why this is so?

Martin