I have heard that one shouldn't reheat a meal with spinach in it. Is there any truth in this and what is the reason? 
Reheating spinach can cause nitrite to be produced. Quote from eufic.org     Spinach and other leafy vegetables contain high concentrations of nitrate. The amount depends on the variety, season, and the soil and water conditions where the vegetable was grown. Nitrate itself is totally harmless, but it can be converted to nitrites, and then to nitrosamines, some of which are known to be carcinogenic. Enzymes present in bacteria convert nitrate to nitrite. This happens especially when spinach is heated, stored and then later reheated. Nitrite itself is a harmless compound, but it should be avoided by infants of up to 6 months. It can affect the ability of the blood to transport oxygen by transforming haemoglobin, the protein that carries oxygen in the blood, into methaemoglobin, a form of the protein which is unable to carry oxygen. This can be dangerous for babies and is commonly known as “Blue Baby Syndrome”. However, in view of the fact that acidic conditions favour the formation of nitrosamines from nitrite, coupled with the facts that nearly all foods contain some protein and are exposed to acid in the stomach nitrosamine production cannot completely be prevented. Hence the recommendation to avoid reheating spinach.  