Camera processing time / buffer screws up (delays) the camera initiating exposure starts quite frequently. On the spectrum this means missed shots.
Most users incorrectly discount camera buffering / processing time as inconsequential.
Have you or one of your friends ever said something to the effect of:
"My camera can shoot 1 billion frames per second in rapid fire mode. I have a Class 50 card that can write 4Tb/s. It isn't my camera. Period. Don't imply my camera could could ever miss a shot. I just spend $XX thousand on it and it is amazing. If you do imply my camera can't shoot 200megapixel at 88 frames per second for 2 years straight, I will get angry."
Yes, yes, there are great cameras and they are getting better all the time. Still, watch out as camera processing, and buffering is still an issue with most, if not all, modern cameras when long exposures are involved.
The reason your camera's processing/write (typically red and on the back) light stays on for a few seconds longer after a long exposure isn't the write time to the card, its the camera, trying to make sense of a very long sampling of data. That takes processing time and happens prior to writing to the card.
If you think you are going to shoot 30 sec exposures with Long Exposure Noise Reduction set to on, continuously (where your camera starts the next shot immediately after the shutter closes) you are setting yourself up for disappointment.
Do your own tests. Cover your eyepiece, put on a lens cap, manually set your exposure to 30 seconds, put your camera into rapid fire mode and hold down the trigger and wait for 5 minutes while timing out your shots. Bonus if you take a cell phone video with sound where you can hear your shutter open and close. Super bonus if you take the audio track into your editor and then document the real intervals and shot timings and share it up here.
Not to ruin the surprise, but after a few shots, the recycling time from shutter close to shutter open will change . . .and increase. This is caused by the processing buffer filling up and your camera not able to start the next shot until there has been enough time to clear part of it.
Understanding how your camera settings affect your image processing time will allow you to play with the setting on the spectrum to maximize your shot counts in long exposure settings.
Most importantly, it will ensure you leave enough time for your camera to process images and prevent ruined shots where your camera can't keep up with your programmed interval times.
Want to play it safe? Always make sure your camera's processing/write light turns off before you trigger the next shot. This typically involves turning off Long Exposure Noise Reduction and choosing an interval that is at least twice your shutter speed.
Think we are full of beans? Prove us wrong for your camera model. We need your help!