A "mega" is twitcher's slang for a very rare bird rarely seen on these shores, usually one seen less than 20X ever in the UK.
A Great Knot (normally seen in Asia in place like China) had been spotted from the south shore of Breydon Water by 2 very reliable observers and I joined a small throng of observers who were watching it. Only 4 have previously seen in Britain. I personally had seen several Great Knots before in China in 1999. The only problem was the bird was very distant, but looking at it through a 60X and 80X telescope, I could not see anything other than a pale grey back, (I could see no dark centered feathering or for that matter any brown on the plumage, but it did show a dark cap) a diffuse high pale grey chest band. In my opinion, the bird just didn't look big enough for a Great Knot. Although it was distant and the only other bird for comparison was an Oystercatcher, which positively dwarfed the bird. I was having serious doubts about the identity of the bird.
Structure wise, the length of the bill appeared equal to the width of the head (Great Knot should have a longer bill) it didn't appear particularly pot-bellied which I remember was a feature I saw on the birds seen in China and the legs appeared to me too short for a Great Knot.
Before you ask, I was looking at the right bird as it was initially pointed out to me by one of the finders.
I am not doubting the original identification made by the finders (they saw the bird much closer) but maybe the bird flew off before myself and the other twitchers arrived and an error was made and we were all looking at the wrong bird, a Knot?