Top 25 network problems

The Top 25 problems as listed by Infobox here are:
  1. Configuration not saved
  2. Saved configurations don't match policy/best practice
  3. Blocked firewall ruleset, unused ACL entry
  4. Firewall connection count exceeded
  5. Link hog - someone downloading music or video
  6. Interface traffic congestion
  7. Link problems and stablity
  8. Environmental limits exceeded
  9. Memory utilization increasing
  10. Incorrect serial bandwidth setting
  11. No QoS
  12. QoS queue drops
  13. Route flaps
  14. OSPF recalculations high
  15. Poor VoIP quality
  16. Routing neighbour changes high
  17. OSPF area not connected to backbone
  18. Unidirectional traffic flow
  19. Router interface down
  20. Unstable to undefined root bridge
  21. Duplex mismatch
  22. Downstream switch
  23. Port in error/disabled state
  24. Unbalanced and unused etherchannels
  25. HSRP or VRRP peer not found
On the download link to the list I have my own network troubleshooting checklist.