This is another entry in the intermittent series I started in October (previous installments are here and here) regarding material that is a giant pain to drill through.
Another material that causes headaches is bedrock which changes drastically and unpredictably over small scales. Examples include small-scale fault zones, which may include significant fractures and breccia zones above or next to relatively massive rock. Or severely mashed-up rocks that have very different properties, such as gneiss on top of pegmatite or coal-bearing seams in mixed shale and sandstone.
Rock coring requires a steady flow of water to cool the drill bit and remove crud that would otherwise jam the core barrel. So drillers bring along drill bits that they think will work the best for what they'll encounter. If they are expecting soft rock and run into a hard layer for a few feet (such as a particularly quartz-rich sandstone), they'll wear down the bit in no time. If they are expecting shale and run into a coal seam, all that soft material will gum up the works and they'll have to keep stopping to pull everything up, clean out the core barrel, and try again. If they're expecting competent (non-fractured) granite and they end up drilling through some zones with loose grains, those sandy bits will cause the core barrel to lock up and then they get to pull up and try again.
I'm not an expert on which drill bits to use for what - CME has a whole catalog that discusses what to select, and individual drillers will have their preferences based on local knowledge. But I have spent hours and hours watching ineffective coring because the units are changing too fast to change out bits or the driller only brought one type in the first place. And it says something that rock coring bits are the most common items in my collection.