Occasional test failure for "block 6"

First observation is that sometime there is a test failure, like in my first "make test" attempt (current Debian testing, amd64):

[yann@yantop:build (master=)]$ make tests
[ 36%] Built target RaptorQ_Static_Profiling
[ 47%] Built target test_c_profiled
[ 52%] Running profiling test...
Block: 0
Dropped 4 source packets
Block: 1
Dropped 4 source packets
Block: 2
Dropped 0 source packets
Block: 3
Dropped 4 source packets
Block: 4
Dropped 2 source packets
Block: 5
Dropped 1 source packets
Block: 6
Dropped 5 source packets
Error: couldn't add the symbol to the decoder
CMakeFiles/profile.dir/build.make:66: recipe for target 'CMakeFiles/RaptorQ_Static_Profiling.dir/src/Graph.cpp.gcda' failed
make[3]: *** [CMakeFiles/RaptorQ_Static_Profiling.dir/src/Graph.cpp.gcda] Error 255

[Edit: that looks vaguely similar to quite https://www.fenrirproject.org/Luker/libRaptorQ/builds/70, although that buid failure does not have the error message]

But then by luck, as "make examples" runs the tests, I endded up rerunning several times, and notice that sometimes shows full success, which prompted me to look closer, and notice that the number of dropped source packets were changing from one run to the other, which does not look like a good property:

$ make examples
[ 33%] Built target RaptorQ_Static_Profiling
[ 42%] Built target test_c_profiled
[ 47%] Running profiling test...
Block: 0
Dropped 1 source packets
Block: 1
Dropped 3 source packets
Block: 2
Dropped 4 source packets
Block: 3
Dropped 4 source packets
Block: 4
Dropped 6 source packets
Block: 5
Dropped 4 source packets
Block: 6
Dropped 4 source packets
Decoded: 501
[ 47%] Built target profile
[ 80%] Built target RaptorQ
Scanning dependencies of target test_cpp
[ 85%] Building CXX object CMakeFiles/test_cpp.dir/test/test_cpp.cpp.o
[ 90%] Linking CXX executable test_cpp
[ 90%] Built target test_cpp   
Scanning dependencies of target test_c
[ 95%] Building C object CMakeFiles/test_c.dir/test/test_c.c.o
[100%] Linking C executable test_c
[100%] Built target test_c
Scanning dependencies of target examples
[100%] Built target examples   

$ make tests
[ 36%] Built target RaptorQ_Static_Profiling
[ 47%] Built target test_c_profiled
[ 52%] Running profiling test...
Block: 0
Dropped 4 source packets
Block: 1
Dropped 1 source packets
Block: 2
Dropped 2 source packets
Block: 3
Dropped 4 source packets
Block: 4
Dropped 4 source packets
Block: 5
Dropped 4 source packets
Block: 6
Dropped 3 source packets
Decoded: 501
[ 52%] Built target profile
[ 89%] Built target RaptorQ
Scanning dependencies of target libRaptorQ-test
[ 94%] Building CXX object CMakeFiles/libRaptorQ-test.dir/test/rfc_test.cpp.o
[100%] Linking CXX executable libRaptorQ-test
[100%] Built target libRaptorQ-test
Scanning dependencies of target tests
[100%] Built target tests

$ make tests
[ 36%] Built target RaptorQ_Static_Profiling
[ 47%] Built target test_c_profiled
[ 52%] Running profiling test...
Block: 0
Dropped 3 source packets
Block: 1
Dropped 2 source packets
Block: 2
Dropped 4 source packets
Block: 3
Dropped 2 source packets
Block: 4
Dropped 0 source packets
Block: 5
Dropped 3 source packets
Block: 6
Dropped 2 source packets
Error: couldn't add the symbol to the decoder
CMakeFiles/profile.dir/build.make:66: recipe for target 'CMakeFiles/RaptorQ_Static_Profiling.dir/src/Graph.cpp.gcda' failed

And then I can also get a different error:

$ make tests
[ 36%] Built target RaptorQ_Static_Profiling
[ 47%] Built target test_c_profiled
[ 52%] Running profiling test...
Block: 0
Dropped 5 source packets
Block: 1
Dropped 3 source packets
Block: 2
Dropped 3 source packets
Block: 3
Dropped 4 source packets
Block: 4
Dropped 4 source packets
Block: 5
Dropped 6 source packets
Block: 6
Dropped 4 source packets
Couldn't decode: 501 - 0
CMakeFiles/profile.dir/build.make:66: recipe for target 'CMakeFiles/RaptorQ_Static_Profiling.dir/src/Graph.cpp.gcda' failed

... but as I rerun it, most often the tests pass!