I have limited bandwidth. I was syncing the most recent few hundred blocks while also trying to watch a youtube video. I was using the icon in the lower right corner of bitcoin-qt to toggle network activity on and off as needed to let the youtube video buffer up.
At one point the youtube video was running out of buffered data so I turned the network off in bitcoin-qt. The icon changed into an 'X', and the tooltip said 'Network activity disabled'. I noticed that bitcoin-qt was still saturating my connection. getconnectioncount showed 1 throughout, and the blocks kept coming. The debug window's 'Network Traffic' tab showed that bitcoin-qt was maxing out my connection, even though the network activity was meant to be 'disabled'.
Eventually the sync finished and the youtube video resumed playing.
This is on Linux, built from the v0.16.1 tag.
I've no idea how it happened, and I cannot reproduce it. But it definitely happened.
Edit: screenshot showing the disabled network icon and recent network activity
