Why does my wireless icon change to a rings symbol? Sometimes on my macbook my wireless icon looks like this. 


In the past I've only seen this when connecting to my iphone's hotspot, and I pressumed it was something along the lines of "we're using bluetooth as well" but when I took the screenshot I was connected to a normal everyday wireless access point.  What causes the change?
That's clearly a tethering icon, so you have three options:


your Mac OS is interpreting things incorrectly from the WiFi chipset (either due to needing a reboot or some systematic error)
the router is sending mixed signals
the hardware is detecting something that is not being sent (error or failure)