I want to share with you some experiences I had to go through last summer, but I've been so busy that I have not had the time to post all of them, I'll start with this one.
During a deployment we faced an issue where a customer with a cisco 5520 wireless controller was complaining about the WLAN being slow or sometimes user got kicked off, it was so random that it was hard to detect the root cause.
The first thing I started noticing on some APs was a high latency in the communications between some APs and the WLC, which for me is really rare, considering they are in the same LAN, at the beggning I thought it could be only those remote sites, since the WLC was in a central location, but after that I found APs with the same high latency even in the same location as the WLC.
Of course that ping responses on the client side were really high, some times even pinging a local gateway
After some troubleshooting when I checked the stats on the WLC's 10 GB ports, I realized there were some FCS errors and were increasing as more traffic was passing through.
So, since the SFPs were new, we decided to replace the fiber jumpers and this is how it looked after that.
So, now you know, a bad fiber can cause erratic behavior on the WLAN, high latency on APs and FCS errors are the way you can detect this.