Ever bought a 'one size fits all' hat, only for it to either cut-off the blood supply to your skull or disappear into the air at the first sign of a stiff breeze?
Annoying, isn't it.
And diets and exercise regimes are exactly the same - what fits perfectly for some can be deeply uncomfortable for others.
Some of your friends might tell you that you need to run three times a week. It helped them achieve their dream body, so why shouldn't it do the same for you?
Others, though, might scoff at such advice and tell you that you should be hitting the weights in the gym instead. And what about yoga? Or Zumba? Or box-ercise?
You could easily face information overload and give up before you've even set a trembling foot on the treadmill.
Which is why a new bit of scientific research has really caught my eye, because it could well be the key to unlocking that near-mythical weight loss treasure chest.
It involves a newly-discovered hormone called 'Irisin', a fat-burning, feel-good chemical released into the body during exercise.
A team of egg-heads from the University of New Brunswick, Canada, are the first to really analyse it while trying to eke out potential human physiological benefits.
And they now suspect that a simple blood test could show a would-be fitness enthusiast exactly which exercises they need to be doing, and for how long, in order to get the best out of their own bodies.
It's a real game changer that signals the end of the typical trial and error, scattergun approach to finding a fitness plan for YOU.
And like all the best ideas, it sounds so obvious you wonder why no-one ever noticed it before!
Physical activity is a science, after all, and any weight-loss programme based upon that science needs to be individualised to cater for a person's needs.
You wouldn't put up with wearing someone's glasses that weren't your prescription, so why should you have to put up with a fitness regime that's not set-up precisely for you either?
This tailor-made approach could guarantee positive results, while also making an immediate impact on the nation's obesity and diabetes epidemics.
Irisin itself - only discovered in 2012 - is a protein which increases the number of calories that are burned, and also helps control blood sugar levels by metabolising glucose.
The university boffins are continuing their research into it for the rest of the year, as they monitor a group of 30 people, between the ages of 19 to 35 and over 60, who all carry extra body weight. Each age range will have roughly the same fitness levels.
The volunteers' blood will be tested at various times during three different types of exercise sessions.
And the scientists will measure how much of the hormone is released, and whether it peaks during or after the exercise.
Using that information, they hope to be able to determine which exercise produces the optimum levels of irisin in each different group of people.
Then it should be a simple matter of translating that lab information and applying it to the masses.
Simply put, the days of you standing in front of the mirror and wondering why you're still fat, despite putting in so much effort, could be well and truly numbered.