I'm not sure if this has been fixed or not, since I ran into it a few days ago, but it's something I noticed.
The virus card's effect can't effectively be canceled. If you activate it, it vanishes, and you're supposed to pick the target. However, if you click cancel at that stage, the virus remains gone.
I'm not sure if there are other issues with monster costs, since I haven't been using a deck that has any to deal with. However, that's also possible.