If you're in Thailand over 90 days you need to report it. There's a 2000 baht fine for not filing. (If arrested its quite a bit more)
Whilst you may not be found out for not filing an eagle eyed IO on your next 90 report may spot that you have missed the august 26 reporting and fine you.
Other than the fine there is normally little repercussion - your choice report or not (and possibly face a fine).
Notes from Immigration :-
- The notification of staying in the Kingdom over 90 days is in no way equivalent to a visa extension.
- If a foreigner staying in the kingdom over 90 days without notifying the Immigration Bureau or notifying the Immigration Bureau later than the set period, a fine of 2,000.- Baht will be collected. If a foreigner who did not make the notification of staying over 90 days is arrested, will be fined 5,000.- Baht and with an additional fine not exceeding 200.- Baht for each day which passes until the law is complied with.