As Eschew Obfuscation points out, I've noticed that a fair number of people put a d where the t should be. But also, I hear many (yes, me too!) pronouncing the word as though it has the d instead of the t. So it's pretty understandable as to why some make that mistake. They're only spelling the word to match the usual pronunciation. Nevertheless, the d is still wrong.

Lots of people put a d in it.

I found this a relatively easy word to spell. The 'u' was confusing for me for a while though. I can understand why people would misspell it.