PageLoad multiple calls
PageLoad was getting called multiple times for a user control in my Master Page. This had me puzzled for a couple of hours while I was looking in the wrong place for the cause. Then I realised what was going on.
IIS URL rewrite module
The site has a news directory that contains news articles, some static content others dynamic. The master page control in question listed the last few news articles and showed a thumbnail of those articles.
You may start to guess what is coming if I say there was also an IIS url rewriting rule to redirect any request that was not found as a physical file to /news/ and thus return the main news page (default.aspx). This had the effect of redirecting users who were using old links to the new front page.
What happened was that development machine had got out of sync with the thumbnail images for this control. Thus when the browser asked for the missing thumbnail, instead IIS did a redirect for that request to /news/. In effect calling the default.aspx for the news directory. As the news front page was sent to the browser instead of the image, the header was rendered as part of this request for default.aspx, resulting in the page header control getting multiple hits depending on how many broken images were present.
This was identified as soon as fiddler was used to look at what the browser was up to. At this point all became very easy to see, several 200 requests for the /news/ url.
The solution for me was to limit the redirect in IIS to be just for .html/.aspx/.htm/.js etc. This solved the problem and properly reported missing images as not found.
Empty image tags
I also noticed a few people talking about empty image tags in server controls causing this same issue. They are correct, even without IIS redirects, there is a redirect in operation, that of serving default.aspx when an empty directory is called. If you have any image tags that render to an empty tag, the browser will unwittingly request default.aspx for the image. This will fire your page events twice or more depending on how many of these links lie in the page. I didn’t see any explanations for this behaviour in those posts so cover it here in case this variation on my issue catches you out.