List has more than 1 row for assignment to SObject on action button in a page from managed package

Question

Installed a managed package, which contain simply 2 pages, controllers, styles, scripts etc. Then created a site using the a page within the package. When i clicked an action button, Received the exception ,

List has more than 1 row for assignment to SObject Error is in expression '{!saveingData}' in page donation:donationform: (Donation) An unexpected error has occurred. Your solution provider has been notified. (Donation)

What will be the reason? Anything regarding the installation or configuration. Any help?
Thanks in advance.

Answer 1

I think you try to execute a SOQL query that returns several results and assign the SObject list returned to a single SObject.

Something like this :

SObject obj = [SELECT ...]; // But SELECT query return more than one SObject

You should do :

List<SObject> objList = [SELECT ...];