Logging Time, light level, temperature, rain, battery voltages and various pin logic levels to a SD card. They are a self powered unit with a sample rate between 1 and 10minutes depending on what we were looking to capture with a battery life in excess of 3 years.
Over those years some interesting problems were found, our general setup was a Uno or a Teensy with a shield for an SD Card/RTC and another shield as a breakout for Analog and Digital inputs we had an interest in recording. These were for installation on site and left often for 3-12 months to log data.
Initially we found a high failure rate of the units, either locking up, powering off or recording random data that simply could not exist. After a lot of testing we found the culprit to be the connectors between the Arduino and the shields being noisy or intermittent, when these were replaced with a soldered pin between the board and shields the problems all stopped. Whilst the time between failure was high, often months apart it did raise concern over the use of that style of plug for reliable operation. Often the problem could also be reproduced with light finger pressure on the connectors.
I only raise this purely to see if anyone else had seen random problems with no apparent reason and whether anyone else has thought on this.