RingCT is mandatory since block 1400000, yet there are 95 non ringct transactions since that block and block 1500000.
User iDunk on IRC suggested that this could be the result of the command sweep_unmixable. Indeed, all these transactions seems to use unmixable outputs. But still, why can't it be a RingCT transaction ?
Example of txs:
RingCT is only mandatory IF it can be used. It cannot be used if there are not enough outputs on the chain to build a ring for the output to be spent, and since all outputs in a ring must have the same amount (either a known amount from pre-RingCT times, or the placeholder 0 for RingCT outputs), it follows that old "dust" outputs will often have unique amounts such as 0.000000065874, with no other outputs with this amount on the chain (or not enough of them). RingCT therefore cannot be used for those ouputs, but we do not want them to become unspendable, so a pre-ringct tx is allowed in that case. Such txes cannot generate more of those odd amount outputs though, so this is not self perpetuating.
