Arhat Culapanthaka's display of psychic powers to a layman (DP Verse 25) In DP Verse 25, we read about Arhat Culapanthaka displaying psychic powers (appearing as many) to possibly a lay person, i.e. the messenger:


  When the messenger from the house of Jivaka arrived at the monastery he found not only one bhikkhu, but a thousand identical bhikkhus.


Now it's popularly known that according to the Vinaya, Bhikkhu are prohibited from displaying psychic powers to lay persons (Kevatta Sutta DN 11). If so, was this display by Arhat Culapanthaka, 


before the vinaya prohibiting such displays,
performed unaware of such a rule, 
exempt from the rule, 
not exempt from the rule, but the intention and effect due to the display of such powers, was of greater benefit than non display.
other
It was not a show for people, that's the first thing you have missed. Here is how it went and what you missed,

The function in doctor Jeewaka's house was attended by the monks of the temple and Ven. Arhat Culapanthaka was at the temple/monastery. As Ven.Arhat Culapanthaka was alone and there was work to be done multiple bodies were used to clean the temple,to sweep it and do some other petty tasks. This is when the messenger from Doc.Jeevaka's house came and saw this.



Sometimes when arahats feel that more harm can come to a society or a person by not revealing his true status the Arahat does some suitable task to prove him that. This was first done by Lord Buddha to tame the ego of "Jatila sages" who thought they are Arahat and Lord Buddha was not. It was done again to tame the ego of the "Shakya cast" who were is relatives of prince siddhartha. 

What we need to understand is that Arahat does not break "Vinaya (laws &amp; guidelines of Buddhism)" no matter what. And that has been clearly stated in Buddhism. So there is no point in Scrutinizing the "Seela" or the conviction of an Arahat because it is not up to petty beings like us. Like always it has happened it should happen by the hands of another Arahat.