square peg in a round hole
nonfit
almost
antimodel
solution in search of a problem
nonaccomplishment
non-starter
nonachievement
frangible
needle in a haystack
imperfection
punch
nonexistent
Scotch mist
lemon
stinker
busted flush
rinky-dink
trash
bottler
nontreasure
mismatch
broken reed
brass farthing
wind egg
stinkeroo
self-defeating
nonstandard
nonsolution
odd one out
lost cause
limitation
unmatchable
all sizzle and no steak
impeller
schlemiel
imperfect
fault
perfectionist
vagarian
dog
no-hoper
cakewalk
rat's nest
weakness
pons asinorum
ticket to ride
antigrav
force of nature
fail-safe
Swiss cheese
subexpert
buster
nontalent
never-would-be
dark horse
dry hole
kink
blunt instrument
unconventional
decelerator
hooker
snap
warts and all
putrescible
no-goer
Heath Robinson
rogue elephant
contaminate
complication
must
flaw
nonmutator
overpower
nonnegotiable
invincible
nonbeing
stopgap
might-have-been
puzzle
esoterica
fragile
fail
pawkiness
mutilate
bubble
gimp
unadept
trinkety
underachieve
imperfectibly
rank outsider
failure
rule out
doozy
obstacle
unworkable
come from a Cracker Jack box