Haven't looked at the full implementation of BigCache, but typically, a cache would fetch the object and key from the data structure. If the full key doesn't match the requested key, then the cache will respond that the key is not in the cache. The calling function would then figure out whether to replenish the cache or not.
Update:
Yes, that appears to be the strategy in shard.go.
if entryKey := readKeyFromEntry(wrappedEntry); key != entryKey {
s.lock.RUnlock()
s.collision()
if s.isVerbose {
s.logger.Printf("Collision detected. Both %q and %q have the same hash %x", key, entryKey, hashedKey)
}
return nil, ErrEntryNotFound
}