A few of my deets:

Version: 23.0.1522.60
Update stream: Stable
System: Windows 8.1 64-bit (WoW64)

Most often this problem occurs when youtube videos are watched. At least I can get it to happen through access only to youtube. I have recently installed BitMeter out of curiosity and have noticed that (once the problem occurs) sitting idle at even only the SpeedDial page, I continue to download/upload packets. The rate varies from ~70kbps to ~1000kbps download, and it only stops when I close Opera fully. Once I let it download for an hour and I went up past 2GB in download.

I have confirmed through my network connections on the control panel that packets are indeed being sent and that it is not just BitMeter's issue. I tested it with Opera 12 and I couldn't reproduce it, but I can reproduce it with 23 quite easily.

Steps that I take:

1. Go to Youtube.
2. Open up a video.
3. Let it play for about 30 seconds.
4. Close every tab but a SpeedDial tab.

The issue starts at step 2, but by step 4 I can easily observe through BitMeter that my download rate is constant but Opera isn't doing anything. Opening more than one youtube video steps the download rate up (so watching a lot of videos ==> download rate is high during idle).
It appears that watching a youtube video causes the problem but I am not sure if it is just my Opera.

Any idea what is going on?