1. How did the Apollo guidance computer handle parity bit errors?
According to Apollo 15 Hardware by Delco Electronics,
Occurs if any accessed word in fixed or erasable memory whose address is $10_8$ or greater contains an even number of "ones." All locations of $10_8$ or greater are stored in fixed or erasable memory with odd parity.
$10_8$ is octal 10 or decimal 8.
This condition triggers an automatic hardware restart:
A RESTART (hardware) and subsequent AGC/LGC Warning is generated for the following alarms:
- Oscillator Failure
- Transfer Control (TC) Trap
- Parity Alarm
- Nightwatchman Fail
- Interrupt (RUPT) Lock
- Voltage Fail
The RESTART inhibits access to memory temporarily, freezes the computer, stores in process information and then transfers control to address 4000. This address has the information address for the next instruction after a RESTART that the software programmer has provided.
2. Were these ever encountered during actual missions?
According to the Apollo Program Summary Report, the most severe anomaly in the entire GN&C system was a transient voltage which gave an erroneous indication to the computer that the inertial attitude reference had been lost. It also states that an open gimbal rate feedback circuit caused unexpected oscillation of the redundant engine gimbal actuator assembly. However, of the computer itself, it unambiguously states:
The performance of the computer was flawless.
I would interpret that as no parity errors.
According to Recovery from Transient Failures of the Apollo Guidance Computer:
In a total of over 25 hours of space flight, the computer has yet to have a transient failure from which the restart feature could be called on to demonstrate its worth.
(credit to @aCVn) That report was published August 1968, before any of the lunar landings.