Build or download an mqlrun binary (version 5.0.0). Prepare a large (at least 1 million docs) BSON file to use as an input (I can provide the one I used to reproduce this issue). Start  mqlrun  from the command line (where  ./tmp  is a directory for spilling-to-disk):  ./mqlrun -f -j -t ./tmp -e '[{$_internalSetWindowFields: {output: {}}}]' big_1000000.bson > output.json  This always causes  mqlrun  to crash with a  SIGABRT : [1] 78171 abort ./mqlrun -f -j -t ./tmp -e big_1000000.bson > output.json The  mqlrun  process also writes the following to the tail end of the JSON output file: { "t" :{ "$date" : "2021-07-20T17:33:45.472Z" }, "s" : "F" , "c" : "-" , "id" :23079, "ctx" : "thread1" , "msg" : "Invariant failure" , "attr" :{ "expr" : "Hit a MONGO_UNREACHABLE!" , "file" : "src/mongo/db/pipeline/process_interface/stub_mongo_process_interface.h" , "line" :271}} { "t" :{ "$date" : "2021-07-20T17:33:45.473Z" }, "s" : "F" , "c" : "-" , "id" :23080, "ctx" : "thread1" , "msg" : "\n\n***aborting after invariant() failure\n\n" } However, when I run  mqlrun  with a  $limit  stage preceding the  $_internalSetWindowFields  stage: ./mqlrun -f -j -t ./tmp -e '[{$limit: 100000}, {$_internalSetWindowFields: {output: {}}}]' big_1000000.bson > output.json  then  mqlrun  runs successfully for limits as high as 100,000. I've tried this with several large files and there's always some point after which  mqlrun  consistently fails with this same error. I've been unable to reproduce this with  $sort  or  $group . I imported the contents of this file into MongoDB using  mongorestore  and am able to run a  $setWindowFields  aggregations without any errors.