The team I'm currently working with want to voluntarily over commit to work in the sprint. So generally we have a pretty good idea what can be taken into the sprint comfortably, and then the team often want to voluntarily (no pressures from the organisation) add a stretch story to the sprint.
Right now its early days on a greenfield project with a single team - but I'm just wondering what the future or current unforeseen perils of this might be.
Currently the only issues I can think of are: