Working at a technical support call centre for an ISP i have many pet peeves.. The greatest on i have is people who install software and don't know what it does, then call in for support.

The two biggest ones are firewalls/routers causing no access and file sharing causing degraded performance.

here is an example of file sharing at it's greatest..
Reply from xxx.xxx.xxx.xxx: bytes=32 time=3855ms TTL=113
Reply from xxx.xxx.xxx.xxx: bytes=32 time=4391ms TTL=113
Reply from xxx.xxx.xxx.xxx: bytes=32 time=3186ms TTL=113
Reply from xxx.xxx.xxx.xxx: bytes=32 time=3809ms TTL=113
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Reply from xxx.xxx.xxx.xxx: bytes=32 time=4350ms TTL=113
Reply from xxx.xxx.xxx.xxx: bytes=32 time=4065ms TTL=113
Request timed out.
Request timed out.
Request timed out.

Ping statistics for xxx.xxx.xxx.xxx:
Packets: Sent = 116, Received = 42, Lost = 74 (63% loss),
Approximate round trip times in milli-seconds:
Minimum = 227ms, Maximum = 4391ms, Average = 1191ms
feel the pain!