What does the "teehee" expression mean? and What is the origin of this facial expression with one's tongue sticking out? both involve expressions in which a character's tongue is sticking out, but I don't think the "teehee" question should be regarded as a duplicate of the other one. Should the "teehee" question stay closed as a duplicate of the other one?
The "facial expression" question comes with specific examples, at least one (if not all) of which are parodies of peko-chan, as Krazer points out. However, I don't think a reference to peko-chan is necessarily something where a character is trying to cover up a mistake they made, which I believe is the expression being asked about in the "teehee" question.
"tehepero" is a word that refers to specifically this latter usage (Japanese source for definition, Japanese youtube video of it being used outside of anime). The "teehee" question may even be too much of a "culture" question (see Where should we draw the line with "culture" questions?) for this site, but if the question stays, I don't think it's a duplicate.
Full disclosure: This issue is complicated by the fact that the tongue of tehepero may have it roots in peko-chan (Japanese source), and even if it doesn't, it's certainly been associated with her (Japanese source 1 Japanese source 2).