I have been a lurker here for 4 years, there is so much great info here that this is the first time I feel like I have something useful to add.
I moved into my current house with a pre-existing in-ground pool 2 1/2years ago. I do my own basic pool care and the water has always looked good but not quite sparkling clear so I decided to do a deep cleaning on the sand filter.
I am confident that the PO never did anything like this to the filter, the water started out looking like coffee with one cream in it! After over 90 minutes (!) it was looking respectable enough that it was time to button it all back up. After a couple hours of running the pump I went out to have a look and there was sand in my pool. I figured I had gotten carried away with the deep clean and damaged a lateral so I got a new lateral assembly
When I removed the old lateral assembly I was shocked to see that the laterals were in fine condition! Then I noticed something at the bottom of the laterals, there is nothing to stop them from extending beyond 90 degrees to the stand pipe which opens a gap at the junction which is plenty big for sand to go through:
The new lateral assembly has a tab to prevent this:
All of that to say, if you have older equipment and are doing a deep cleaning be very careful you do not accidentally push a lateral down and cause a gap.