This seems to have happened on a pipelines callback with the following query string:
{
    "callback_params": [
        "{\"server_query_interval_seconds\": 60, \"deadline\": 1507021238.6194501, \"step_name\": \"renderer_side_navigation_content_browsertests\", \"task_id\": \"38f52254306e8010\", \"step_name_no_platform\": \"renderer_side_navigation_content_browsertests\", \"task_started\": true, \"task_completed\": false, \"call_args\": [\"chromium.linux\", \"Linux Tests\", 62776, \"renderer_side_navigation_content_browsertests\", \"DumpAccessibilityTreeTest.AccessibilityWindowCropsItems\"]}"
    ], 
    "pipeline_id": [
        "3d9ee1a6b2774f6f9de417ca50a8e3eb"
    ]
}

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/1b1c847691481788cbe08e7ba76008f371db9811

commit 1b1c847691481788cbe08e7ba76008f371db9811
Author: Jeffrey Li <lijeffrey@chromium.org>
Date: Tue Oct 10 21:50:52 2017

[Findit] Flake Analyzer - Reset swarming tasks instead of deleting

Flake swarming tasks going through DetermineTruePassRatePipeline are
deleted between each iteration. Instead of deletion, they should be
reset. Based on the existing logic, _NeedANewSwarmingTask for
trigger_flake_swarming_task_pipeline should always return true and thus
override the one in trigger_base_swarming_task_pipeline.

Bug:  770759 
Change-Id: I6b5766300d2f0bddcb7ad4e8b8b285439704ae6d
Reviewed-on: https://chromium-review.googlesource.com/703197
Commit-Queue: Jeffrey Li <lijeffrey@chromium.org>
Reviewed-by: Chan Li <chanli@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Reviewed-by: Brandon Wylie <wylieb@chromium.org>

[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/trigger_base_swarming_task_pipeline.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/process_flake_swarming_task_result_pipeline.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/test/trigger_base_swarming_task_pipeline_test.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/flake/recursive_flake_pipeline.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/trigger_flake_swarming_task_pipeline.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/flake/test/determine_true_pass_rate_pipeline_test.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/model/base_swarming_task.py
[modify] https://crrev.com/1b1c847691481788cbe08e7ba76008f371db9811/appengine/findit/waterfall/flake/determine_true_pass_rate_pipeline.py