One way would be to call whatever <tab> is normally calling instead of issuing the tab key press. Something like:
inoremap <expr> <TAB> pumvisible() ? "\<C-y>" : "<esc>:call NormalTabFunction()<cr>a"
The command that actually worked in this specific case was (resulting from the discussion in the comments):
guess you need autocompleter+snippet plugin, since Supertab will be slow when the tags file are large, recommand you two group plugins:
(1) YouCompleteMe + UltiSnips
(2) Neocomplete + clang_complete + Jedi + neosnippet + neosnippet-snippets
They are the best C/C++/Python completer+snippet plugin combinations I ever used, but incompatible with each other.