Hi, I'm currently designing a new error box to NeedFreelance.com (the old one wasn't good). As header I have the choice to write "Oops.. An error has occured!" or "Error!" I like the "Oops.. An error has occured!", but is it too frivolous? Big sites like digg.com, myspace.com also write such "funny" things, but on my site, NeedFreelance.com, I don't know if my users will like it. You can see the error box at http://needfreelance.com/register.php?step=3
Peronsally, I dont see anything wrong with the "Oops" but ultimately, its down to what your end users are like. Although, you would need to be pretty stuck up to complain about an error message like that.