If you are accessing files, EAFP is more reliable than LBYL, because the operations involved in LBYL are not atomic, and the file system might change between the time you look and the time you leap. Actually, the standard name is TOCTOU – Time of Check, Time of Use; bugs caused by inaccurate checking are TOCTOU bugs.
Consider creating a temporary file that must have a unique name. The best way to find out whether the chosen file name exists yet is to try creating it – making sure you use options to ensure that your operation fails if the file does already exist (in POSIX/Unix terms, the O_EXCL flag to open()). If you try to test whether the file already exists (probably using access()), then between the time when that says “No” and the time you try to create the file, someone or something else may have created the file.
Conversely, suppose that you try to read an existing file. Your check that the file exists (LBYL) may say “it is there”, but when you actually open it, you find “it is not there”.
In both these cases, you have to check the final operation – and the LBYL didn’t automatically help.
(If you are messing with SUID or SGID programs, access() asks a different question; it may be relevant to LBYL, but the code still has to take into account the possibility of failure.)