This is an eyepiece-fitting solar filter:
This is the set-up I used to video the results. The Sun was at an altitude of about 55 degrees in a hazy sky; i.e. this wasn't even full midday sunlight. The telescope is a home-made "comet-catcher" refractor (yes, the aperture cap is a baseball cap -- well, if the cap fits...).
After 2 minutes and 54 seconds, this happened:
Straight on, it looked like this:
The solar filter looked like this:
...and the other side:
The crack in the filter appeared, suddenly and without warning, after less than 3 minutes. Although a crack like this will not transmit sufficient heat to cause thermal injury to the retina (unless you are really stupid and keep your eye there without even blinking, despite the painful glare), it will pass sufficient UVA to cause photochemical retinopathy. Although it is usually reversible (typically between 2 months and 1 year), blindness from this sort of photochemical injury can be permanent.