Text size changing underneath mouse pointer
It might seem like a nice idea to have text become bold, or even larger when you position the mouse over it. But this can cause problems when the layout of the page is affected.
Suppose I position my mouse over such text, and it gets bigger. This can cause the text to be moved away from the mouse pointer, which means that the text reverts to its original size. Once the text has shrunk back to its original location under the mouse, it then grows and the cycle repeats until the mouse is repositioned.
My suggestion is to only do this if you can be sure you will not be affecting the layout of the web page. For example make sure the text is bounded within a box that is large enough for the text, regardless of whether it is enlarged by the position of the mouse.
Redirecting to an error page
Occasionally websites have problems, and so it is appropriate to display an error to the user. Rather than having the desired web page display an error, some websites redirect to a dedicated web page that explains an error has occurred.
Errors quite typically are due to a temporary glitch, and so you might expect to be able to use the refresh button in your browser to reload the web page. If you've been redirected to a separate page this isn't possible, even clicking back will redirect you to the error page again.
Instead it seems necessary to click back in rapid succession, before the redirection kicks in. That way you can once again attempt to repeat the steps required to load up the problematic page. This may involve filling in a form once again, but either way this is not user friendly! It would be preferable to display error messages on the web page the error occurred, so that it's possible to simply click refresh.