List.Sort exception - Unable to sort because the IComparer.Compare() method returns inconsistent results

Question

While trying to perform a List sort, I am getting the following error:

Unable to sort because the IComparer.Compare() method returns inconsistent results. Either a value does not compare equal to itself, or one value repeatedly compared to another value yields different results.

Please suggest what might be the likely reasons behind this issue.

Stack Trace:

Message: Unable to sort because the IComparer.Compare() method returns inconsistent results. Either a value does not compare equal to itself, or one value repeatedly compared to another value yields different results. IComparer: 'Premise.PremiseLIVE.CommGateway.Workflows.Sequential.SequentialWorkflowData.TransportJobPriorityDataSort'.
StackTrace: at System.Collections.Generic.IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer(Object comparer)
at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer`1 comparer)
at System.Collections.Generic.List`1.Sort(Int32 index, Int32 count, IComparer`1 comparer)
at Premise.PremiseLIVE.CommGateway.Workflows.Sequential.WorkflowActivities.wcSortTransportJobs.ExecuteSequential(ActivityExecutionContext executionContext)
Source: mscorlib
TargetSite: Void ThrowOrIgnoreBadComparer(System.Object)"

Code:

IComparer<JobPriorityData> comparer = null;
comparer = new TransportJobPriorityDataSort( orgTransportTyps, transportPriorities, false );

if ( comparer != null )
{
    jobList.Sort( comparer );
}

private int _xLTy = -1;
private int _xGTy = 1;
private int _xEQy = 0;

public TransportJobPriorityDataSort( OrgTransportTypCollection orgTransportTyps, TransportPriorityCollection transportPriorities, bool ascending )
{
    _orgTransportTyps = orgTransportTyps;
    _transportPriorities = transportPriorities;

    _ascending = ascending;
    if ( !_ascending )
    {
        _xLTy = 1;
        _xGTy = -1;
    }

    _currentTime = DatabaseTimeHelper.DatabaseUtcTime;
}

public int Compare( JobPriorityData x, JobPriorityData y )
{
    try
    {
        // past-due jobs prioritized over not-past-due.
        if ( ( x.TransportPickupTime.Value <= _currentTime ) && ( y.TransportPickupTime.Value > _currentTime ) )
        {
            return ( _xGTy );
        }
        if ( ( x.TransportPickupTime.Value > _currentTime ) && ( y.TransportPickupTime.Value <= _currentTime ) )
        {
            return ( _xLTy );
        }

        // either: (A) both jobs are past-due or (B) neither job is past-due.

        // Order by Typ
        if ( _orgTransportTyps[ x.JobTransportType ].AssignSeqNbr > _orgTransportTyps[ y.JobTransportType ].AssignSeqNbr )
        {
            return ( _xLTy );
        }
        if ( _orgTransportTyps[ x.JobTransportType ].AssignSeqNbr < _orgTransportTyps[ y.JobTransportType ].AssignSeqNbr )
        {
            return ( _xGTy );
        }

        // then by pickup time
        if ( x.TransportPickupTime < y.TransportPickupTime )
        {
            return ( _xGTy );
        }
        if ( x.TransportPickupTime < y.TransportPickupTime )
        {
            return ( _xLTy );
        }
    }
    catch ( Exception )
    {
    }

    return ( _xEQy );
}