In following the ongoing Brexit drama I've seen claims that the first vote was unfair because it was only for yes/no and didn't say anything about what form an actual Brexit would take. This uncertainty obviously makes it hard for the British leadership to negotiate with the EU, since they don't know what their electorate actually wants.
Why doesn't the UK hold a second referendum to clarify what the electorate wants? The second referendum does not have to ask the same question as the first (leave/remain), but could ask for example whether the country should accept a hard Brexit if the EU refuses to compromise, or what areas it desires Theresa May to focus on most (e.g. trade, immigration, Northern Ireland, etc).
Edit: six months on, the UK parliament has held votes on what I had in mind. Questions such as "customs union with EU?", "Common market 2.0?" and "Second stay/leave referendum?" are all yes/no questions that can be answered by a referendum.