case doesn't save if second case opened

to reproduce:

1. open contact1 and click to create a case – but don't save
2. open contact2, and create a case (including saving it)
3. return to contact1 and attempt to save the case – errors. also, if you change the case type, it will attempt to pull the case2 record via ajax (where it would be pulling the case custom fields).

although this is a slightly unusual workflow, I don't think it is that uncommon – staff receives a call and begins opening a case. they get a second call, switch contacts and file the case, then return to complete the first.