In roughly the order
1. Network Engineers (What's this command do?)
2. Power failures (What's this switch do?)
3. Cable cuts (Backhoes, enough said)
4. Hardware failures (What's that smell?)
5. Congestion (More Bandwidth! Captain, I'm giving you all she's got!)
6. Attacks (malicious, you know who you are)
7. Software bugs (Your call is very important to us....)