Nigel Clough was frustrated to see his side come within seconds of another comeback victory at Bramall Lane on Saturday.
Romain Vincelot headed home in the 99th minute to rescue a point for ten man Leyton Orient, after late goals from Paddy McCarthy and Marc McNulty, following Jobi McAnuff's dismissal, looked to have won the game for the Blades.
But Clough resfused to be too downbeat, after witnessing another domiant display from his players, despite falling behind to a Jay Simpson goal in the first-half.
The Gaffer remarked: "First of all we were delighted with the performance, for the second home game on the bounce we have played some brilliant football and created a host of chances.
"We have missed certain opportunities but also the opposition have ridden their luck, we hit the post, the bar, they have cleared a couple of the line and we were denied a blatant penalty in our opinion.
"But the character we displayed against a side unbeaten on the road for a very long time was excellent again, just a bit of naivety in the final 60 seconds of injury-time cost us two valuable points.
"The main threat from a team with just ten men is set-pieces, so to give away a free-kick and then a corner so close to the end is really frustrating but we'll learn from the experience.
"We have benefited a lot from late goals, including two on the day, but ultimately one at the other end with the final kick has cost us and that's cruel."