I have noticed that words such as careful, beautiful or sinful are often misspelled as carefull, beautifull or sinfull. A spell checker will often flag such mistakes. However, in scientific writing, there are many technical expressions not recognized by the spell checker anyway, so that such mistakes will remain unnoticed. The best example I can think of is spinful, which is often misspelled as spinfull.
Let us look at the origin of this mistake in more detail. First, the suffix -ful has nothing to do with the word full. That is, something that is beautiful is not full of beauty. Instead, -ful indicates that something has beauty, as opposed to something that has no beauty. Another good example is careful, the opposite of careless (rather than carefull and careempty). Similarly, spinful means “with spin”, as opposed to spinless (“without spin”). Other examples include helpful, harmful, and doubtful. A particularly interesting case is fulfil, which some of you may be tempted to spell as fullfill.
While full is contained in expressions such as full-time or full-scale, there is no English word which the suffix -full. For German native speakers, the distinction between full and ful is complicated by the fact that, for example, careful is translated as rücksichtsvoll, with voll being the translation of full.
There is also an interesting difference regarding pronunciation between full and –ful. According to the Longman Pronunciation Dictionary, -ful is usually pronounced as just [fl], as opposed to the word full [fʊl] (listen here), although [fʊl] is also acceptable. Hence, helpful is typically pronounced as [ˈhelpfl] rather than [ˈhelpfʊl], listen here.