In lightweight frame construction, inside corners of showers and tub surrounds are movement joints and should be caulked instead of grouted.

The easy fix is to cut out the crack and caulk the joint.

The hard fix is a topical or otherwise ineffective repair that may allow water infiltration and unseen damage down the road.

Mongo

What type of caulk should it be? Won't it look funny to have one joint caulked and the rest of the grout lines with sanded grout?

Thanks mongo. Can you tell me what to use to "cut" the grout out?

You have to realize that water can penetrate the grout whether there is a crack in it or not - and even if it is sealed. Tile and grout are NOT what waterproofs your shower. The waterproofing should be behind the tile.

For a DIY repair?

Manual tools include a simple grout saw, something along the lines of this:

To something powered, like a diamond bit chucked in a dremel or rotary cutoff tool:

In a shower you simply want water shedding down the face of the tile and grout. Behind your tile is a tile backer board of some sort. Not knowing the age or construction of your shower, this could be anything: greenboard, cement board, gypsum core board with an acrylic facing, or a floated wall of mud. With these types of construction, your tile and grout are your first line of defense in preventing water intrusion. Behind those tile backers should be a water shedding membrane of some sort; 6-mil poly or tar paper on the walls, and CPVC/CPE membrane on the floor pan and lapped up the lower 10" of the walls. That is your final layer of protection, your only true "waterproofing". When cutting out grout you only want to cut the grout, you don't want to cut so deep that you go through the backer and into your waterproofing membrane.

In a shower with a topical waterproofing membrane, where you have either a sheet membrane (like Kerdi) or a trowel on membrane (like Redgard) right behind the tile, you don't want to cut full-depth into the grout joint as you don't want to damage that topical membrane.

In showers with a topical membrane like Kerdi, cracks in grout are more cosmetic than anything else, for water that gets through cracks in the grout still will not penetrate the Kerdi.

In showers with a membrane buried behind the cement board, or with a CPVC or CPE liner buried in the middle of the shower pan, I'm more concerned as I want no unnecessary water getting into and wetting the tile backer board or cementicious substrates in the walls/floors. In that case I'm more protective of the grout on the walls or the caulked inside corners, I want to keep that first line of defense in top-notch condition.

The difficult thing is not knowing how the shower was constructed, and even more importantly, how well the detailing was done. That's why, even though tile and grout are not "waterproof" per se, you still want to protect the integrity of that first line of defense as best you can.

Mongo

Ours is new construction...11 months old. The showers are on a concrete slab. The method (as best as I recall) was a heavy liner, covered by Hardi backer board, then the wall tiles were set directly onto that. The floor of the shower had some sort of sloped mud that hardened prior to the tiles being set.

Mongo, thank you for the info.