It actually happens quite often, sometimes to DCA and not DL and sometimes to DL and not DCA, basically here's how it works:
First they stop selling one day, one park tickets (at this point if
Disneyland is closed, they may sell a one day pass that states something like "Valid for Admission to California Adventure Only until 4:30 PM" After 4:30 PM, if DL's capacity is back to normal you would then be able to enter Disneyland, but not re-enter DCA. They don't do this part all of the time)
Next they stop selling tickets all together
Then they go to allowing only Resort Guests and Annual Pass Holders
Then they go to letting one guest in for each guest that leaves.
Then they completely stop allowing people to enter, although re-entry may still be possible, depending on the situation.
This happens:
End of April (when the So Cal promotion ends, usually to DCA)
Mid June (When most schools get out to either park)
the week of July 4th (Disneyland)
Various days throughout August (either park, never seems to happen to both at once though)