Hello!

I've noticed that Ktorrent "misbehaves" when there are system-wide proxy is configured in KDE.
I configured the system proxy to use tor socket proxy (127.0.0.1:9050), other proxy settings (HTTP, HTTPS, etc) are blank.

1) To avoid sending all traffic via tor, I clear the checkbox "Proxy > Use KDE proxy settings" (all other Ktorrent proxy settings are blank, actually, including "Use proxy for tracker connections"). Although no traffic is sent via tor this way, HTTP trackers report "Error: Invalid data from tracker". UDP trackers and P2P connections (both downloading and uploading) seem to work fine.
2) If I check "Use KDE proxy settings", all traffic goes via tor, naturally.
3) If I set the system proxy settings to "No Proxy", Ktorrent works without any problems (HTTP trackers are "OK").

So perhaps querying HTTP trackers actually uses system proxy, but perhaps, not the (configured) socket proxy, but a blank HTTP proxy. Or something between those lines.

I'm using ktorrent 4.3.1 (KDE 4.10.4, Kubuntu 12.04).

I suspect that your HTTP queries are indeed still using the proxy. Unfortunately this is something which KIO does not readily allow applications to change or control - so KTorrent might not be able to do much about this. (It would need features being added to KIO and for the HTTP protocol support in KIO to respect that).