What are the sources of error for the combustion of acetylene gas

Question

 4 test tubes have been filled with different amounts of acetylene. One completly full , half full and so on. The one with less acetylene produced a complete combustion. What are the sources of error in this lab( not human errors)?

Answer 1

This is probably not a complete answer, but it must be noted that under compression acetylene can undergo an exothermic dimerizattion to form vinylacetylene.  From there other hydrocarbons can form.   Neglect of this possibility might lead to errors in the analysis.