In the first episode with Martha Jones (S3:E1), in the first few scenes of the show, he takes off his tie and drops it, completely unknowing of his future to return and retrieve it.
Later, to show Martha he can travel in time, he goes back and picks the tie of the floor. Unless we're just having to accept that tie stayed completely dry all day and in that position, we can assume that The Doctor basically did something he was against -- visit his own timeline -- seconds within the drop, to pick it up and return. This is also risking being seen, and Time Lord would recognise the Tardis sounding.
Regardless of the state of that tie (holding the suspension of disbelief), that was an incredibly risky move. While it holds up to the feel of Tennant's magical, quick, exciting Doctor, it's not very Doctor-like.
Why would a time-lord, who consistently travels around time constantly writing parts in his image (or stopping others from ruining that image), go back on his own time-line, just to return and show Martha? It seems like insanity.