An example from Hexagon where things went wrong:
%R0<def> = L2_loadrigp <ga:@fp04> ; load function address J2_callr %R0<kill>, ..., %R0<imp-def> ; call *R0, return value in R0
ScheduleDAGInstrs::buildSchedGraph would visit all instructions going backwards, and in each instruction it would visit all operands in their order on the operand list. In the case of this call, it visited the use of R0 first, then removed it from the set Uses after it visited the def. This caused the DAG to be missing the data dependence edge on R0 between the load and the call.
The final result was that the load and the call were packetized together, which meant that the call used the prior value of R0 instead of the loaded one.