I don't think Sam deliberately messed up the meal, but I could totally understand if she did. Feeling as isolated as she obviously is, nobody to talk about her feelings to, no control over how she's perceived by the others - this could have given her a bit of personal satisfaction that at least she can control some things that go on in the house.
I was surprised that she failed the task though, giving others more amunition to have a go at her about - but maybe it was for the above reasons too.
In the DR Uncut show, she looked so upset saying it was scary how Lesley hated her so much for no reason. Maybe she's deliberately giving them a reason
I have to say, Sam has become one of my favourites in there (not much competition I know) just for the way she's sticking with it, not letting herself get too upset and for not having walked. I think she's the strongest person in the house - who else could've handled what's been thrown at her like she has?