EventLogging is reveiving events for the schema MobileWikiAppSavedPages that do not validate because the required field appInstallID is missing.
Check the required field here:
https://meta.wikimedia.org/wiki/Schema:MobileWikiAppSavedPages
Examples follow:
eventlogging_processor-client-side-events.log.1.gz:2015-03-01 22:40:01,813 (MainThread) Unable to decode: ?%7B%22schema%22%3A%22MobileWikiAppSavedPages%22%2C%22revision%22%3A10375480%2C%22event%22%3A%7B%22action%22%3A%22savenew%22%7D%2C%22wiki%22%3A%22dewiki%22%7D; cp3021.esams.wikimedia.org 72571809 2015-03-01T22:40:01 91.198.174.91 WikipediaApp/4.0.6 (iPhone OS 8.1.3; Tablet)
Decoded string: {"schema":"MobileWikiAppSavedPages","revision":10375480,"event":{"action":"savenew"},"wiki":"dewiki"}
eventlogging_processor-client-side-events.log.1.gz:2015-03-01 22:40:10,681 (MainThread) Unable to decode: ?%7B%22schema%22%3A%22MobileWikiAppSavedPages%22%2C%22revision%22%3A10375480%2C%22event%22%3A%7B%22action%22%3A%22savenew%22%7D%2C%22wiki%22%3A%22frwiki%22%7D; cp3021.esams.wikimedia.org 72572029 2015-03-01T22:40:10 91.198.174.91 WikipediaApp/4.0.6 (iPhone OS 7.1.2; Phone)
Decoded string: {"schema":"MobileWikiAppSavedPages","revision":10375480,"event":{"action":"savenew"},"wiki":"frwiki"}
eventlogging_processor-client-side-events.log.1.gz:2015-03-01 22:40:23,425 (MainThread) Unable to decode: ?%7B%22revision%22%3A10375480%2C%22event%22%3A%7B%22action%22%3A%22savenew%22%7D%2C%22schema%22%3A%22MobileWikiAppSavedPages%22%2C%22wiki%22%3A%22enwiki%22%7D; cp3022.esams.wikimedia.org 74690040 2015-03-01T22:40:23 91.198.174.92 WikipediaApp/4.0.6 (iPhone OS 8.1.3; Tablet)
Decoded string: {"revision":10375480,"event":{"action":"savenew"},"schema":"MobileWikiAppSavedPages","wiki":"enwiki"}