The most common mistakes in sensor applications include failing to select the most appropriate sensing principle/technology for the task. Below are some case studies where we were able to help teams find the best sensor for their need.

Need Sensors
Measure blood oxygen level as part of a device to detect sleep apnea and correct it Wrong Sensor: A reflective pulse plethysmograph, while attractive because it is small, low cost, and can be placed almost anywhere on the body, is not accurate enough to provide medically acceptable results without expensive and inconvenient calibrating. Using this sensor, would not provide accurate data that could be used for medical diagnosis.
Right Sensor: A transmissive pulse plethysmograph, while accurate, is restricted to use on parts of the body that are thin enough to allow light to pass through. A design change allowed them to place it around a finger.
Desire to measure core body temperature non-invasively Wrong Sensor: A simple thermistor based measurement of the temperature of exposed skin
Right Sensor: Depending on the requirements, there are a many way to measure core temperature non-invasively. Infrared sensors can now read the temperature of arteries through thin layers of skin and can do so with very good accuracy. Heat flow sensors have also shown some good promise as well. There is also the less convenient under the armpit approach, which has been commercialized for many years. A simple temperature sensor on the skin, even at the wrist, may be able to infer core body temperature at times, with sensor fusion and a clever algorithm. There are times when the skin temperature is close to the core temperature, but you have to know the level of activity of the person, the air temperature, and if there is perspiration on the skin. All of this can be measured with sensors.
Heart rate measurement that is convenient and always available. Wrong Sensor: While ECG electrodes provide a very good way to measure heart rate, they can be quite inconvenient. It is critical to have good contact, which often requires moist ECG pads. This is not acceptable for most consumer products and many medical devices. Large flexible electrodes can make good contact, but they usually require some adhesive, which needs to be replaced periodically.
Right Sensor: A pulse plethysmograph picks up pulse as well as measuring blood oxygen. Even the reflective type can be used to measure pulse rate, so it can be even placed on the wrist where a transmissive pulse plethysmograph would not work. While it will not provide accurate Oxygen measurement, especially during movement, it will provide an adequate measurement for many applications, and it can provide an accurate heart rate even during movement and provide better comfort and convenience. The extra comfort and convenience will encourage usage.

Image source: Ion Chiosea