Consider this beamer file:
\documentclass{beamer}
\begin{document}
\begin{frame}
\begin{tabular}{p{1cm}p{1cm}p{1cm}}
aaaaaa & \only<2>{aaaaaa} & \onslide+<2>{aaaaaa}
\end{tabular}
\end{frame}
\end{document}
This input causes slide 2 to look like this:

Why are the cells not aligned, and what can I do to fix this?