It is not really a slip up. He is a scarecrow
with straw for legs. When he gets down he is
unsteady on his feet. Straw legs are suppose
to make you walk unsteady. He has never walked
at all being hung up as a scarecrow. So he is
unsteady on his feet.
Ray Bolger (the scarecrow, for those who don't know)added that in himself. Yes, it was entirely on purpose. The director liked it, so he let him keep it. Ray and Bert Lahr were constantly ad-libbing. Bert added the line "unusual weather we're havin', ain't it?" when it began to snow in the poppy field. They were always trying to out-do each other.
Upon watching the movie once again last night...it is interesting to note that during the infamous Tin man scene, when they dance off, Scarecrow falls not once but TWICE. Once again...he's a Scarecrow, he doesn't know how to walk (as he never has) which they expand on throughout the entire length of the film. In the Motown remake "The Wiz" they also expand on that over and over. For you to think he actually ACCIDENTALLY fell, though he had done the SAME thing numerous times prior to that scene, is completely ridiculous...and most importantly--NOT a slipup.