When using net-snmp tools (for example with the tool snmpwalk), Debian / Ubuntu users often get errors like

Bad operator (INTEGER): At line 73 in /usr/share/mibs/ietf/SNMPv2-PDU
Undefined OBJECT-GROUP (diffServMIBMultiFieldClfrGroup): At line 2195 in /usr/share/mibs/ietf/IPSEC-SPD-MIB
Undefined OBJECT-GROUP (diffServMultiFieldClfrNextFree): At line 2157 in /usr/share/mibs/ietf/IPSEC-SPD-MIB
Undefined OBJECT-GROUP (diffServMIBMultiFieldClfrGroup): At line 2062 in /usr/share/mibs/ietf/IPSEC-SPD-MIB
Unlinked OID in IPATM-IPMC-MIB: marsMIB ::= { mib-2 57 }
Undefined identifier: mib-2 near line 18 of /usr/share/mibs/ietf/IPATM-IPMC-MIB
Expected "::=" (RFC5644): At line 493 in /usr/share/mibs/iana/IANA-IPPM-METRICS-REGISTRY-MIB
Expected "{" (EOF): At line 651 in /usr/share/mibs/iana/IANA-IPPM-METRICS-REGISTRY-MIB
Bad object identifier: At line 651 in /usr/share/mibs/iana/IANA-IPPM-METRICS-REGISTRY-MIB
Bad parse of OBJECT-IDENTITY: At line 651 in /usr/share/mibs/iana/IANA-IPPM-METRICS-REGISTRY-MIB

This is due to a combination of errors: some IANA MIBs have been updated since snmp-mibs-downloader 1.1 was released (see http://www.iana.org/assignments/), and some IETF files are broken.

To fix these issues, firstly let's run

download-mibs

Now, we need to update a few files (please note that my modified versions of SNMPv2-PDU and PATM-IPMC-MIB are not IETF official files! Use them only if you know what you are doing).

sudo wget http://www.iana.org/assignments/ianaippmmetricsregistry-mib/ianaippmmetricsregistry-mib -O /usr/share/mibs/iana/IANA-IPPM-METRICS-REGISTRY-MIB
sudo wget http://pastebin.com/raw.php?i=p3QyuXzZ -O /usr/share/mibs/ietf/SNMPv2-PDU
sudo wget http://pastebin.com/raw.php?i=gG7j8nyk -O /usr/share/mibs/ietf/IPATM-IPMC-MIB

The remaining errors

Undefined OBJECT-GROUP (diffServMIBMultiFieldClfrGroup): At line 2195 in /usr/share/mibs/ietf/IPSEC-SPD-MIB
Undefined OBJECT-GROUP (diffServMultiFieldClfrNextFree): At line 2157 in /usr/share/mibs/ietf/IPSEC-SPD-MIB
Undefined OBJECT-GROUP (diffServMIBMultiFieldClfrGroup): At line 2062 in /usr/share/mibs/ietf/IPSEC-SPD-MIB

seem to have been resolved with net-snmp 5.7.x