What is the mechanism of the reaction of ethyl prop-2-ynoate to ethyl (2Z)-2,3-dibromoprop-2-enoate?
A search of SciFinder will also produce the formation of the (E)-dibromide 3. The conditions described for the formation of the (Z)-dibromide 2 are taken from a paper by Myers1 as reported earlier by Tippett.2 Rossi3 has reported the formation of the (E)-dibromide 3 using pyridinium hydrobromide perbromide. 
Given the more vigorous conditions for the formation of the (Z)-isomer 2 compared to the milder conditions for the formation of the (E)-isomer 3 suggests that the former reaction is under thermodynamic control while the latter is kinetically controlled.  There is the distinct possibility that bromine radicals may effect the isomerization via species 4 as may surreptitious acid.  



1) A. G. Myers, M. M. Alauddin, M. A. M. Fuhry, P. S. Dragovich, N. S. Finney, and P. M. Harrington, Tetrahedron Letters, 1989, 30, 6997.
2) R. G. Hall and S. Trippett, Tetrahedron Letters, 1982, 23, 2603.
3) F. Bellina, A. Carpita, M. De Santis and R. Rossi, Tetrahedron Letters, 1994, 35, 6913.