Reading I Fear Our Mobile Group Being Forced To Follow Scrum crystallised in my mind what can go wrong when you treat Agile as a methodology. It describes a team successfully using kanban which is to potentially be required to use scrum — because that's becoming the company standard.
Making a team follow an agile methodology is exactly *not* Agile.
Agile is "Individuals and interactions" being valued more highly than processes. Imposing Scrum looks like valuing the process more than the team.
Agile is "self-organising teams" and letting "the team [reflect] on how to become more effective, then tune and adjust accordingly." Imposing conformity on a team that has already adjusted is a backwards step; you're asking a team that has optimised somewhat for the individuals in the team to de-optimise again.
This doesn't mean that you can't teach an agile team anything. The manifest starts with "We are uncovering better ways of developing software by doing it and helping others do it." A team that can't be corrected, or won't learn better ways, isn't agile. For that matter, a team that won't learn in any walk of life has started the downhill path to decline.
For what it's worth, I'm sure that a competent lean team that tries Scrum for a while will learn from it, even if they end up optimising back to something more fluid.