Common sense says it had to have happened at least once.  Well, it did:

A
man executed by the state of Texas in 2004 was convicted on an
erroneous interpretation of fire evidence, according to a report from
four leading arson experts.

[snip] 

“Each
and every one of the indicators relied upon have since been
scientifically proven to be invalid,” the Arson Review Committee report
states.

[snip]

John J. Lentini,
the former chairman of the forensic science committee of the
International Assn. of Arson Investigators, led the review panel and
said in an interview that he was convinced that Texas had executed an
innocent man.

One of the many ill-conceived
arguments pro-death penalty advocates use is that nobody had ever
proved that an innocent person was legally executed in the United
States.  Not anymore.

Fire is a truly complex and poorly-understood subject.  When I used to work at CSI,
we had a number of discussions about doing an arson case and really
digging into the relevant forensic examination techniques.  But we
never did, at least while I was there.  One big reason was that on
close scrutiny, most of the “techniques” amounted to little more than
unscientific guesswork, more a series of beliefs and habits than any
peer-reviewed analysis, really just one notch up from finding witches
with dunking chairs.  (The other reason we never did such an
episode — probably the main one, as I understand it — was that fire
is really super-hard and crazy expensive to shoot.)

It turns out
— big shock! — that fire behaves in unexpected ways.  Many of
the rules of thumb that arson investigators have used for many years
turn out to be complete crap. 

“Arson is the
only crime for which you can be executed based on the opinion of a man
with a high school education,” Lentini said, referring to the fact that
many arson investigators are qualified by judges as “experts” even
though they lack scientific training.

Back when
Chimpus Maximus was governor, you recall, he signed death warrants with
a bloodthirst unparalleled in modern U.S. history.  If somebody
out there has time to go through the cases looking for arson, there may
be some important history to correct.

Texas has the highest percentage in the country of people imprisoned on arson convictions.

Human
beings make mistakes.  So do judicial systems.  Capital
punishment inevitably involves the murder of a small number of innocent
people. 

Anyone who pretends otherwise is a liar.