Pie or Crumble

Fortunately nothing more than an old wives tale and utter gobshite. I have had both together and the only ill effect that I suffered was an overwhelming need for more. Fortunately sated by eating some more.

A shameless cut and paste:

All parts of the rhubarb plant contain oxalic acid, a substance which, in combination with some nutrients (especially calcium salts), can be difficult to digest. However, the amount of oxalic acid present in rhubarb is small enough that it won't normally cause any problems. Pineapples contain a range of acids but hardly any oxalic acid; mixing rhubarb with pineapple will increase the overall amount of acid you are eating, but will barely increase the amount of oxalic acid at all. The only possible detrimental effect of adding pineapple to rhubarb could come from the strong enzymes in pineapple that break down proteins. If you hold fresh pineapple against your gums for a few minutes they tingle - the enzymes are eating into the proteins of your gums. There is a small amount of protein in the rhubarb's cell walls; if this is destroyed by the pineapple, it could make the cells weaker and easier to break, letting the oxalic acid out more easily. So you might taste more oxalic acid, but you wouldn't ingest any more. I don't think this is an issue.

http://www.guardian.co.uk/lifeandstyle/2006/feb/24/foodanddrink.features112