NetTraffic Monitor is an app from Android Market which claims to be capable of monitoring your data traffic. It seemed quite weird to me even from the first run, because it requested me super user access (I'm running on a rooted device), even though its only work was to view and count my traffic, and nothing more.
After I installed it, I observed that the internet over 3G/2G networks was no longer working (the device was sending data, but it wasn't able to receive back anything). Actually, I didn't realize that the problems started after I installed NetTraffic Monitor, it took me about 2 days to realize that (during that time, I spoke with my carrier, thinking that they had problems with the network).
The app indicated only about 30 kb during that two days on mobile data networks, and absolutely no traffic on WiFi, even though I probably had about 50 MB on it. So I decided to uninstall it because it was useless (I also realized then that the problems started after its installation), and after the uninstall, guess what? The internet started working normally, and I haven't got any problems since then.
The app is the first one if you search for "Traffic Monitor" in the Market, so just be careful what you install and what you don't.