Custom water cooling loops are works of art when they're done right. Glass tubes, RGB pumps, a radiator the size of a small country. The build log photos look like a product shoot. The person who built it is justifiably proud.
And then there is the leak test. The part where you fill the loop with distilled water and coolant, let it run, and wait to see if physics respects your fittings.
It does not always respect your fittings.
The threads linked here are from people who discovered this the hard way — some before they powered on (the smart ones), and some after (the expensive ones). No permanent damage was done in the making of most of these posts. Mostly.