I can duplicate the problem with BIAB2011.

If I keep pressing F4, the song will wait for a midi signal or spacebar,
but every once in a while it will play the 1st note, then wait.

I never see the problem later in the song though, only at the beginning.
If I stop somewhere later in the song with the spacebar, and restart it
with the spacebar, it always waits as it should.

As you noted, a false midi signal would not play only one note. And a false
spacebar would always have to be in pairs (two space characters) which seems
very unlikely.