That happened when I was generating coverage report for 35 unit test binaries:

$ tools/code_coverage/coverage.py courgette_unittests gpu_unittests headless_unittests audio_unittests media_unittests media_blink_unittests media_mojo_unittests media_service_unittests net_unittests services_unittests service_manager_unittests skia_unittests storage_unittests blink_heap_unittests wtf_unittests blink_common_unittests angle_unittests pdfium_unittests accessibility_unittests gfx_unittests gl_unittests keyboard_unittests snapshot_unittests views_unittests wm_unittests url_unittests breakpad_unittests content_unittests mojo_common_unittests sql_unittests unit_tests cc_blink_unittests crypto_unittests pdf_unittests swiftshader_unittests \
>   -b out/coverage \
>   -o total_report \
>   -c 'out/coverage/courgette_unittests' \
>   -c 'out/coverage/gpu_unittests' \
>   -c 'out/coverage/headless_unittests' \
>   -c 'out/coverage/audio_unittests' \
>   -c 'out/coverage/media_unittests' \
>   -c 'out/coverage/media_blink_unittests' \
>   -c 'out/coverage/media_mojo_unittests' \
>   -c 'out/coverage/media_service_unittests' \
>   -c 'out/coverage/net_unittests' \
>   -c 'out/coverage/services_unittests' \
>   -c 'out/coverage/service_manager_unittests' \
>   -c 'out/coverage/skia_unittests' \
>   -c 'out/coverage/storage_unittests' \
>   -c 'out/coverage/blink_heap_unittests' \
>   -c 'out/coverage/wtf_unittests' \
>   -c 'out/coverage/blink_common_unittests' \
>   -c 'out/coverage/angle_unittests' \
>   -c 'out/coverage/pdfium_unittests' \
>   -c 'out/coverage/accessibility_unittests' \
>   -c 'out/coverage/gfx_unittests' \
>   -c 'out/coverage/gl_unittests' \
>   -c 'out/coverage/keyboard_unittests' \
>   -c 'out/coverage/snapshot_unittests' \
>   -c 'out/coverage/views_unittests' \
>   -c 'out/coverage/wm_unittests' \
>   -c 'out/coverage/url_unittests' \
>   -c 'out/coverage/breakpad_unittests' \
>   -c 'out/coverage/content_unittests' \
>   -c 'out/coverage/mojo_common_unittests' \
>   -c 'out/coverage/sql_unittests' \
>   -c 'out/coverage/unit_tests' \
>   -c 'out/coverage/cc_blink_unittests' \
>   -c 'out/coverage/crypto_unittests' \
>   -c 'out/coverage/pdf_unittests' \
>   -c 'out/coverage/swiftshader_unittests'