Encountered this weird error from MSBuild today:

Unhandled Exception: Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: MSB0001: Internal MSBuild Error: Global Request Id 6 has not been assigned and cannot be retrieved.
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Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: Global Request Id 6 has not been assigned and cannot be retrieved.
   at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Object[] args)
   at Microsoft.Build.Shared.ErrorUtilities.VerifyThrow(Boolean condition, String unformattedMessage, Object arg0)
   at Microsoft.Build.BackEnd.SchedulingData.GetScheduledRequest(Int32 globalRequestId)
   at Microsoft.Build.BackEnd.Scheduler.HandleRequestBlockedOnInProgressTarget(SchedulableRequest blockedRequest, BuildRequestBlocker blocker)
   at Microsoft.Build.BackEnd.Scheduler.ReportRequestBlocked(Int32 nodeId, BuildRequestBlocker blocker)
   at Microsoft.Build.Execution.BuildManager.HandleNewRequest(Int32 node, BuildRequestBlocker blocker)
   at Microsoft.Build.Execution.BuildManager.Microsoft.Build.BackEnd.INodePacketHandler.PacketReceived(Int32 node, INodePacket packet)
   at Microsoft.Build.BackEnd.NodeManager.RoutePacket(Int32 nodeId, INodePacket packet)
   at Microsoft.Build.BackEnd.NodeProviderInProc.RoutePacket(Int32 nodeId, INodePacket packet)
   at Microsoft.Build.BackEnd.NodeEndpointInProc.SendData(INodePacket packet)
   at Microsoft.Build.BackEnd.InProcNode.OnNewRequest(BuildRequestBlocker blocker)
   at Microsoft.Build.BackEnd.BuildRequestEngine.RaiseRequestBlocked(BuildRequestBlocker blocker)
   at Microsoft.Build.BackEnd.BuildRequestEngine.IssueBuildRequest(BuildRequestBlocker blocker)
   at Microsoft.Build.BackEnd.BuildRequestEngine.IssueUnsubmittedRequests()
   at Microsoft.Build.BackEnd.BuildRequestEngine.EngineLoop()

Turns out I had a <Target /> with a DependsOnTarget attribute which referenced a target that didn’t exist (because I’d forgotten to <Import /> the related .targets file in which it was defined!)