Well, wouldn't they be guys who did little in camp, and had very small roles in the season?
We are talking about the idea of keeping a guy solely for the future in fear of losing him to waiver claims. There seem to be a reasonable amount of examples of this, and I don't see any that worked out.
Crable, Price, Tate, McKenzie(IR), Ornberger come to mind off the top of my head.
Crable - Drafted 3.78 in 2008; released at end of training camp in 2010.
Granted he was then signed to the practice squad and promoted to the 53-man roster in 2010, so perhaps in hindsight the Pats should have simply waive/injured and reached an injury settlement in 2009.
Price - Drafted 3.90 in 2010; released Dec 3 2011
Pats should have given up after one season? Easy to say in hindsight, but considering his late start due to graduation class dates and when he could first start practicing, a little bit of leeway seems to have been warranted. If anything this seems to be an example of a short leash, and not the opposite.
Tate - Drafted 3.83 in 2009; released at end of camp in 2011
When the NFL moved the kickoff back, Tate's role became diminished and the Pats released him. In 2010 Tate did have 432 yards receiving and 3 TD, so he wan't completely unproductive, despite coming off a late season-ending injury that resulted in surgery.
McKenzie - Drafted 3.97 in 2010; released at end of camp in 2010
How much quicker than that should the Pats have cut ties with McKenzie? On the first day of rookie minicamp, when he blew out his knee?
Ohrnberger - Drafted 4.123 in 2009; released in February of 2012
Pats gave him three years, a case could be made in which that was one year too may.
On the other hand he has played 26 games in the last two seasons.
Overall I'm not seeing that these are very good examples of players being given too much time to develop. Push the needle too much to that side and you end up like the Dolphins or Chargers giving up on Welker, or the Jets giving up on Woodhead early in their careers - or, to go further back in franchise history, the Pats giving up on Steve McMichael, a third round draft pick from a generation ago.