I squeeze tube watercolor paint into my palette and allow them to dry completely before I use them. After a while, some colors tend to dry harder than other colors. The plastic palettes flex a bit, with traveling or even just moving them around the studio, it's rare but sometimes a hardened color will pop out of its well.
There's a simple solution to replacing a popped color:
1. Don't discard the popped color.
2. Clean the empty well with a wet brush and then wipe it out with a tissue or damp sea sponge.
3. Using clean water, brush across the bottom and sides of the hard paint.
4. Press firmly back into place.
Allow it to dry and you're back in business. Wetting the bottom of the dried paint will cause it to get sticky enough to stick back in place.