Yes, for not registering for NS. Give a call to CMPB and check with them your status. They will advise you. Number of factors to be considered for severity of punishment. Age left, current age, procedure followed or not followed etc. Previously they will not say anything but now they will give you ball park outcome.
If agreeable, they will advice you what to do such as providing date of arrival etc
Those who left Singapore prior to 11 yrs then (now 13 yrs) will require to register for NS and then seek deferment followed by rennoucing of citizenship by 21. If this has not been done, its an offence. Those who left after that age, definately have to serve.
my singapore passport has expired many many years ago, for that reason I took up foreign citizenship to facilitate international travel