When constructing a custom, non-sealed Geiger?M?ller tube in a chamber in which you can vary the gas mix and pressure, pay attention to sharp edges of electrified parts outside of the tube itself: you ensured there was no sharp edges inside the tube, so avoid points where the electric field is stronger, thus avoiding inadvertent ionization of the gas mix.
When the same potential is applied to parts outside the tube, also in the chamber, sharp parts (e.g. a nut and the end of a piece of wire) can cause strong electric fields to be produced. This can cause a fun, purple glow (( The gas in the chamber was mostly argon. )) from one’s apparatus. This is bad.
Fortunately the high-voltage power supply is current-limited, and no damage took place.
In related news, electrical tape is extremely useful at insulating (amazingly enough) electrical devices and can stop the aforementioned problem.