I noticed frequently that some site say that I already visited it... Or that I am currently online, or some other strange stuff like that...
I guess that someone is also using the same internet IP as my network, or someone is on my network... How I check all computers using that same IP? (or computers on my network... or... you get it :P)
EDIT: Clarifications: I know the internet IP is random, what I mean is that those "sharing" sites for example (like rapidshare, megaupload, whatever...) may say that my IP is already downloading something. Or when connecting to a IRC network, it may block my connection saying that I am a "clone" (the term some IRC networks use for someone connecting several times).
My network structure: There are a radio antenna that grabs the internet from the ISP, they don't allow me to fiddle with the modem... Then a router distributes the internet using both wireless and cables. I guess either the ISP is sharing that IP with several people, or someone is tapping on the wireless.