Mistakenly sealing an envelope before it was fully packed doesn't mean you have to tear it open and use a new one. Freeze a sealed envelope so it can be opened and resealed without causing noticeable damage. The cold, dry environment of the average home freezer is ideal for hardening the adhesives used to seal most envelopes. With the freezer method of opening sealed envelopes, your error will be your secret.
Slide the sealed envelope into a zip-top plastic freezer bag and press out the excess air as you seal the bag. The plastic will protect the paper from ice crystals and food smells.
Place the zip-top bag containing the envelope in the freezer and close the door securely. Remove the bag from the freezer after a stay of three hours or more.
Take the sealed envelope out of the bag and immediately slide a dull blade or letter opener under the flap to break the adhesive seal. In a few minutes, the envelope will reach room temperature and can be resealed.
Refill the envelope and re-wet the adhesive on the flap backing with your tongue or a damp sponge. Press and hold the damp flap firmly for 30 seconds to set the adhesive.