Word or phrase for bumbling through the least optimal course
I'm sure I've seen this employed as a comedic device in various films/cartoons. An inept character goes through a sequence of actions over a short timespan, each of which is the worst possible at that point, and seemingly by accident. And yet, the timing and accuracy are such that a highly skilled person would be hard pressed to have enacted the same course if they had tried on purpose.
"Yes, after stepping on the loose marble and knocking into each vase, you managed to flail in precisely the right way as to send a spoonful of mustard directly into your boss' eye, and then fall backwards onto the one spot in the room occupied by the wedding cake. And how were you to know, as you cast about for something to grip to help yourself up, that the curtain would come undone and fall into the fire and become tangled in your jacket buttons such that, as you attempted to flee the conflagration, you brought it into contact with all (and only) the most flammable objects in the hall? It was a simple case of _______"
It is precisely explained in the trope disaster dominoes which falls under domino effect.
Basically, instead of a single mess-up, the character manages to chain a lot of them into a bigger one. Slipping on the Banana Peel while holding a two-by-four, hitting someone behind him holding a lit cigarette who lands in a pool of gasoline... etc. Usually ends with the site of said mess-up being completely destroyed (and/or Stuff Blowing Up).
If someone sets off (or claims to have set off) Disaster Domino(e)s on purpose, it may be Exactly What I Aimed At.
http://tvtropes.org/pmwiki/pmwiki.php/Main/DisasterDominoes
The domino effect definition from dictionary.cambridge.org:
the situation in which something, usually something bad, happens, causing other similar events to happen
The phrase a comedy of errors, from William Shakespeare's play by the same name, has come to mean something very like this. From Wiktionary:
(idiomatic) A set of amusing or farcical events involving a series of awkward missteps or other mistakes.
Of course in the original play the errors were literal mis-takes, as identical twins are confused for one another, but the phrase has come to be used for everything from bumbling ball handling to (probably criminal) professional negligence to a series of misadventures when trying to obtain a wedding cake. Your example is so extreme that you could probably tack on some superlative qualifier, like "a comedy of errors of epic proportions", but I think the phrase would work either way.
Another, more recent phrase that has been used to describe everything going wrong all at once, in the worst possible way, is a perfect storm; however, I think this phrase has more implications of some sort of multiplicative effect—the individual problems somehow make one another much worse than they would have been in isolation—rather than the serial effect of your description. This is backed up definitions of the term, such as this description from Wikipedia:
A perfect storm is an event in which a rare combination of circumstances drastically aggravates the event.1 The term is used by analogy to an unusually severe storm that results from a rare combination of meteorological phenomena.
Finally, for a comedically understated expression, you could also use the literal set-phrase a series of unfortunate events. Just keep in mind that this venerable expression has gained new life and new connotations from the children's book series of that name by Lemony Snicket (AKA Daniel Handler).
a simple case of ineptness or clumsiness
OR
a string of bad luck