Do you think it's better to overstay? Because according to jcqh's calculations, the fares for overstaying is sometimes better than tapping in and out many times.
One example (for me):
Tap in at Yishun -> Tap out at Jurong East: 44¢
Tap in at Jurong East -> Tap out at Chinese Garden: 39¢
Tap in at Chinese Garden -> Tap out at Jurong East: 39¢
Tap in at Jurong East -> Tap out at Serangoon: 49¢
Tap in at Serangoon -> Tap out at Yishun: 49¢
Total: $2.20
If overstay:
Tap in at Yishun -> Tap out at Yishun: $2 (fine) + 29¢
Total: $2.29 (saving the time to tap out and in, and missing trains)