NRF said they'll ask about your modules most likely and that you should revise contract law pre-interview if you're a Law student. That being said the trainee I spoke to on their open day said she didn't actually get asked about it. One of the partners said he also likes to ask "what's your favourite case" and it catches lots of people out because they didn't expect it and they just start waffling. Also for their negotiation exercise, Law students are grouped together and non-Law are grouped together, so the level of knowledge expected is different.
Linklaters have asked "what is an indemnity" "what is a warranty" before
Lots of firms don't ask any legal knowledge Qs at all.