What Bitt describes will alter the image with no chance of ever getting back what you had originally. There's no question that quitting the program will destroy any cached image data that might be used for resampling while the program is running.

What Mark describes is Image Resize which is not the same thing as Image Transform with manual resize. They don't even use the same resampling algorithms within the application.

It's conceivable that an application like Photoshop could cache an original layer or element such that when you resize it, the resampling is done from the information in that cache. However, I doubt very much this is the case - and it's easy to prove. Photoshop fails miserably at many basic functions/levels that I simply can't believe the engineers would ever have the desire, let alone the engineering know-how to implement something that worked properly that way. Seriously, Photoshop is a pretty basic application that relies on so much garbage legacy code that it would make your head spin. Don't get "basic" confused with "simple" - it's definitely convoluted and crufted up enough to be anything but simple.

Take a simple image that you've created yourself, preferably not continuous tone (not a photo). A crop of a screen-capture showing part of a desktop window would be good, maybe even with some text. Grab about 100x100 pixels. Paste it into a blank Photoshop document larger than the image (like 400x400) with a transparent background. Paste another copy so now you have two. Move one of the copies up to one of the corners for safe keeping. Select the other image and press CTRL/CMD-T for transform. Look at the Info palette to confirm the dimensions. Hold down shift to maintain aspect as you resize and change the scaling to some oddball value like 142% or 191%. Press RETURN to apply the change. Press CTRL/CMD-T again for transform. Again hold shift as you resize and look at the info palette so you can scale the image back to its original pixel dimensions. Press RETURN to apply changes.

Now look at the two images. They're different.

Always keep source images safe in alternate files.

Also, if you're working on anything but photos, use Illustrator instead of Photoshop in practically every case. It does a better job of rasterizing and usually a better job with resampling and using the Save For Web feature (changing color depth, compression, color remapping, etc..) In fact I even use Illustrator for isolating and resizing product photos. I will import/place a PSD at full resolution into an Illustrator document, making sure it's tagged as 72dpi to make sure no resizing is done. Then I will crate an outline to clip it from its background - the path tools/UI in Photoshop are practically useless. Then I'll use Save for Web on the clipped image to save off scaled versions with transparent backgrounds.

EDIT (added):
In fact, Illustrator doesn't do a final rasterization until you save out to a bitmap format. So you're free to resize a placed bitmap all you want within the document itself because it's only showing you a placeholder. A Photoshop-like application could definitely be designed to work this way, allowing you to save files in a composite format that would simply store each pasted or drawn bitmap object/layer at its original pixel dimensions while rendering to screen at the re-targeted dimensions. Exporting to some other non-native format would then resample and blend all the objects/layers according to your preferences and design/layout.