Nothing bad, in my opinion it's always a good tshoot exercise, here a collection of the funniest tshoot issues I recently played with.
4th place for: "BGP doesn't work"
that was a nice trouble, trying to get bgp working on an old netscreen. The ScreenOs documentation at the moment is little or not helpful at all, so I finished solving thanks to the "ScreenOs Cookbook".
By the way, you have to enable bgp under the interface on ScreenOs, otherwise won't work.
3rd place for: "Fix my trunk"
a simple trunk between two switches wasn't working. With CDP enabled, the two switches wasn't even listed as cdp neighbors, but both ports were up and STP was placing the ports in FWD state. After asking them to change cables and ports, realized that probably one switch had some software crash. Reloaded the one with uptime 1 year+, all worked like a charm.
2nd place for: "Don't touch my QoS"
during a NAT configuration on a 800 series, the owner said something like "don't touch my qos, now voip is working very well". Was something like "class-map VOIP, match access-group 180, service policy blah, class VOIP, priority 8000000, interface dialer, service-policy output blah". The line speed was 8Mbps in total, and the access-list 180 was missing. I must admit, I obeyed, haven't touched it.
1st place for: "can't ping"
a /24 subnet with only one server and a netscreen firewall as gateway. Was awesome to find the ".0" ip address assigned on the netscreen interface, something like: "interface ethernet3 ip 192.168.0.0/24"