two_SIG0 = {SIG0;SIG0}

This sequence matches two consecutive occurrences of SIG0. The first attempt at time 100 fails immediately, because SIG0 is low, resulting in a red spike. The second attempt begins to match at time 200, and then succeeds at time 300, since SIG0 is still high at time 300. A new attempt is also begun at time 300, creating a transaction record that overlaps with the transaction record that started at time 200, so the time 300 transaction is placed on an overflow signal (two_SIG1_1). SIG0 goes low before time 400, so the time 300 transaction fails at time 400. A new transaction is also began at time 400, which fails immediately, resulting in a red spike at time 400 on two_SIG0.