This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...hat’s only a one-time score. This is mostly what happened to Voorhees’ ShapeShift earlier this year. Miners changed their policy in regards to transaction fees, deleting transactions with fe
es below a certain value, while ShapeShift continued to accept them.
read full book
The fact that ShapeShift failed to stay up-to-date with changes in miner policy, and paid a small price for doin
g so, is hardly evidence of unconfirmed transactions being ‘trivially double spent’ or ‘always unsafe’. Yet the opportunity was not missed to push that propaganda on everyone.