Adhoc testing is not like formal testing with test plan or 
test case.It is a random and free style testing performed 
on application without using test case document.It is 
carried out in the following manner,
1.After the completion of all test case excution before 
they are going to relase the build.
2.If sufficient time is not available to test the 
application according to test case.
3.Testing whether the application is working or not.

whereas
Error guessing is basically guessing an error by looking at 
the application, which will be done by a experienced person.
EX:Blank or null character in string,negative 
value,observing the cosmetic defects etc.

Error guessing : Error guessing is a technique of finding 
error or guessing that error will exists based on past 
experience or based on Domain knowledge


Adhoc Testing : It is similar to exploratory testing.
testing done on the overall functionlaities with intution 
of learning and checking whether the application is working 
or not according to the requirements.