When the nurse decided to participate in the deception of keeping Romeo and Juliet's marriage a secret it was well intentioned on her part. The nurse assisted in the marriage because she wanted to see Juliet married. The nurse treated Juliet like her own daughter and wanted her to get married without waiting two years. The nurse wasn't thinking logically to support two people who had only just met, get married. This well intentioned act turned out to be fatal to both of the characters.
Friar Lawrence's decision to marry Romeo and Juliet and deceive their families was based on his well intentioned belief that the marriage would end the feud between the Montegues and the Capuets. When Romeo went to Friar Lawrence's cell to ask him to marry he and Juliet, the Friar had some serious doubts. However, he chose to ignore those doubts and take a chance that this union would create peace between the families. The Friar planned to marry them, inform the families at a later date, and have everything end up right in the end. But once again, a well intended act turned out to be destructive.
Juliet's deception about taking poison to fake her own death had the most tragic consequences of all. The intention of her plan was to create the illusion that she had died and then run off with Romeo to live happily ever after. This deception backfired, and it cost both Romeo and Juliet their lives. Others who knew her secret plan kept silent and also participated in this deceptive act which ultimately ended in destruction.
A little secret here, a small deception...