How is it possible that the f2 generation offspring had some short plants when the f1 generation had no short plants on mendel's expirement?

The f1 generation may have genes for short plants just that the genes for long plants are dominant over the genes for short plants(recessive) so when cross pollination happen and that two f1 long plants which are homozygous cross pollinate, the recessive genes have a chance of being paired up in new offspring(F2 offspring) thus the possiblity of short plants in f2 generation

For example, the cross between two pure plants (homozygous), being a dominant characteristic over another, in case high on short.

High = AA
Short = aa

F1 = AA x aa = 100% Aa = high = heterozygous

F2 = Aa x Aa
 
      A    a
A  AA  Aa
a  Aa  aa

Therefore, of this crossing, we have
75% High plants (AA e Aa)
25% short plants (aa)

This is possible because the heterozygote plant had the short plant allele.