Honestly it can happen and does happen. We are an organic business and never spray our flowers. We also actively encourage as much insect life as we possibly can. Any stowaways are usually pollen beetles, tap them out of the box and either release them or despatch them. They are completely harmless, carry no diseases and cause no damage to the flowers which are still safe to eat. If the weather has been particular close then there might be thunder flies lurking in the petals particularly with flowers such as elderflower and any of the daisy family which they are particularly fond of. Environmental Health suggest that ebible flowers should be washed before use. This has to be done incredibly carefully as it can damage the integrity of the flower structure and petals but is an option if you are concerned.