Non-consecutive steps

The reaction mechanism does not necessarily consist of a sequence consecutive steps. Apart from the trivial case where consecutive steps are written in random order, some more interesting possibilities are 

One or more steps have been written backwards  

The mechanism has been written such that all steps except one has the form 

Parallel steps convert the same reactants into the same products through different routes. 

In a reaction mechanism, short-lived intermediates may be formed by some steps and consumed by other steps. The mechanism may contain intermediates, which have not been observed experimentally. 

---